Method for forming bumps, semiconductor device, and solder paste

ABSTRACT

The present invention relates to a method for forming bumps on a substrate provided with electrode pads. The method includes providing a mask having openings corresponding to the electrode pads, filling each of the openings with a solder paste, and heat treating the solder paste, wherein the solder paste includes solder powder. Preferably, the solder powder contains no more than 10 wt % of particles whose diameter is greater than the thickness of the mask and no more than 1.5 times this thickness. Preferably, the solder powder contains no more than 10 wt % of particles whose diameter is not less than 40% the diameter of the opening portions, or no less than 30 wt % of particles whose diameter is 40 to 100% the thickness of the mask.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for forming bumps onelectrode pads provided on a substrate, to an electronic component onwhich bumps are formed, and to a solder paste.

[0003] 2. Description of the Related Art

[0004] There has been a growing need for higher mounting density withelectronic components in recent years, and bare chip mounting methodshave been attracting attention. There are two types of bare chipmounting method: a face-up method involving wire bonding, and aface-down method featuring metal bumps. Face-down mounting is becomingmore and more prevalent today. A benefit of connecting with metal bumpsby face-down method is the lower resistance of the connection. On theother hand, numerous demands are imposed on this method, such as lowercost, ensuring a precise bump height in order to achieve stableconnection reliability, and forming bumps at a fine pitch correspondingto the electrode pads of a semiconductor chip.

[0005] Plating and vapor deposition are just two conventional bumpformation methods. These bump formation methods require a tremendousequipment investment, and make it difficult to control bump height andmetal composition, among other problems. In view of this, engineers havebeen taking a closer look at printing, which allows a metal paste to besupplied at low cost.

[0006] One type of printing method makes use of a metal mask. Inaddition, as disclosed in JP-A-7-273439 and JP-A-11-340270 andelsewhere, there is also a method that utilizes a resin mask. When ametal mask is used, one in which openings have been formed correspondingto the locations where the electrode pads are formed is placed over asubstrate. When a resin mask is used, a resin layer is formed over asubstrate, after which the portions corresponding to the electrode padsare removed to form openings. The two methods are similar in that afterthis, a squeegee is used to push a solder paste applied over the maskinto the openings and thereby form bumps. When a metal mask is used, itis removed after the openings have been filled with the solder paste,but when a resin mask is used, it is removed as needed after the bumpshave been formed.

[0007] However, if a large proportion of the solder powder that makes upthe solder paste has a large particle diameter (such as an averageparticle diameter of 30 to 40 μm), there tends to be variance in thesize of the bumps that are formed. Causes of this include the fact thatsome of the solder powder that has filled the openings is wiped awaywhen the squeegee is moved back and forth over the mask, and that whenthe metal mask is removed after the openings have been filled with thesolder paste, the solder paste clinging to the inner walls of theopenings ends up being taken away with the mask.

[0008] To avoid this problem it is necessary to use a solder powder witha small proportion of particles whose diameter is large. For instance,it is good to use a solder powder with a large proportion of particleswhose diameter is no more than {fraction (1/3)} the thickness of themask (when the thickness of masks commonly in use is considered, this issubstantially a particle diameter of 15 μm or less).

[0009] Meanwhile, methods for producing a solder powder include discatomizing and gas atomizing. With these methods it is difficult tostably produce a powder with a small particle diameter. Accordingly, thecurrent approach is to produce a powder having a particle sizedistribution within a certain range, and then separate and collect thefines. However, not only does separating out the fines requireconsiderable labor, it is also difficult to collect a large quantity offines. For instance, with existing technology a solder powder of 20 μmor less only accounts for about 20% of the total powder, which is alsodisadvantageous in terms of cost. Also, because a fine powder with asmall particle size has a larger specific surface area and is thereforeoxidized more readily, the solder paste made up of this solder powderhas a shorter life.

SUMMARY OF THE INVENTION

[0010] The bump formation method provided by the first aspect of thepresent invention is a method for forming bumps on a substrate providedwith a plurality of electrode pads, comprising the steps of providing amask having a plurality of openings corresponding to the plurality ofelectrode pads, filling each of the openings with a solder paste, andheat treating the solder paste, wherein the solder paste contains solderpowder and a flux vehicle, and the solder powder contains no more than10 wt % particles whose diameter is greater than the thickness of themask and no more than 1.5 times this thickness.

[0011] Unless otherwise specified, the term “substrate” as used in thepresent invention includes all substrates on which electrode pads areformed, which of course includes circuit substrates and silicon wafers,but also includes semiconductor chips and so forth. When an opening isnot circular, “open diameter” refers to the diameter of a circle havinga surface area equivalent to the surface area of the opening.

[0012] The solder paste used in this bump formation method must have asmall proportion of solder powder with a relatively large particlediameter as compared to the thickness of the mask. This reduces thedanger that the solder paste filling the openings will be wiped awaywhen the mask is coated with the solder paste and a squeegee is thenmoved back and forth over the mask in an effort to pack the insides ofthe openings with the solder paste. Also, when a metal mask is used,there will be less danger that the solder paste clinging to the innerwalls of the openings will be taken away with the metal mask when themask is removed after the openings have been filled with the solderpaste. Accordingly, there will be less variance in the bumps if they areformed by the above method.

[0013] The smaller is the quantity of solder powder within theabove-mentioned particle diameter range, the more pronounced this effectwill be, and the ideal proportion for such solder powder is therefore 0wt %. For the above effect to be realized even better, it is preferableto use no more than 10 wt % solder powder having a particle diameter of40% or more of the open diameter of the openings.

[0014] In a preferred embodiment, the solder powder contains at least 30wt %, and preferably at least 50 wt %, particles whose diameter is 40 to100% of the mask thickness.

[0015] This solder paste has a larger proportion of solder powder ofsuitable particle size as compared to the mask thickness, and a smallerproportion of solder powder of relatively small particle size. If thethickness of the mask is about 50 to 100 μm, for example, then theproportion of solder powder having a particle diameter of 20 μm or lessis small. As discussed above, it used to be that preparing a solderpowder having a particle diameter of 20 μm or less not only was laborintensive, but also produced a low yield and was expensive, but if theproportion of solder powder with such a particle diameter is reduced,then these drawbacks are automatically ameliorated. Also, if theproportion of solder powder with a small particle diameter is small, thesolder powder as a whole is not as susceptible to oxidation, so anotheradvantage is a longer life for the solder paste.

[0016] The average particle diameter of the solder powder as a wholeshould be suitably determined as dictated by the thickness of the mask,the diameter of the openings formed therein, and so on, but is 5 to 20μm, for example.

[0017] One or more elements selected from the group consisting of tin,lead, silver, antimony, bismuth, copper, indium, and zinc can be usedfavorably as the solder component that makes up the solder powder, forexample. More specifically, 63% Sn—Pb (melting point: 183° C.), Sn-3.5%Ag (melting point: 221° C.), 5% Sn—Pb (melting point: 315° C.), and thelike can be used to advantage.

[0018] Meanwhile, the flux vehicle can contain rosin, an activator, anda solvent.

[0019] The primary role of the rosin is to increase the adhesion of thesolder paste. A variety of known rosins can be used, examples of whichinclude polymerized rosin, hydrogenated rosin, and esterified rosin.

[0020] The primary role of the activator is to remove the oxidation filmformed on the surface of the electrode pads or on the surface of theindividual solder powder particles when the solder paste is heattreated. An organic acid or an organic amine can be used, for example,as this activator. This is because, in most cases, an organic acid hascarboxyl groups in the skeleton of the molecular structure, while anorganic amine has amino groups in the skeleton of the molecularstructure, so both are able to remove the oxidation film from the solderpowder surface and the electrode surface in the solder paste.

[0021] At least one type of organic acid or organic amine selected fromthe group consisting of sebacic acid, succinic acid, adipic acid,glutaric acid, triethanolamine, and monoethanolamine is used as theactivator. For the action of the activator to be maximized, it ispreferable to use one that decomposes or vaporizes near the meltingpoint of the solder. Meanwhile, at temperatures below the melting pointof the solder, the activator in the solder paste must be uniformlydispersed in the paste in order for its oxidation film removal effect tobe maximized, so the use of one that is miscible with the solvent orrosin is preferred. Accordingly, when an Sn—Ag-based solder is used, forinstance, the use of sebacic acid (decomposition temperature: 230 to290° C.), succinic acid (decomposition temperature: 200 to 250° C.),adipic acid (decomposition temperature: 230 to 280° C.) or the like ispreferred.

[0022] The amount of activator contained in the solder paste is 0.1 to 2wt %, for example. If the activator content is too high, it will lead toelevated viscosity of the solder paste, the fluidity of the solder pastewill suffer, and it will be difficult to fill the openings in the mask.On the other hand, if the activator content is too low, the oxidationfilm cannot be sufficiently removed from the solder powder, etc.

[0023] The primary role of the solvent is to adjust the viscosity of thesolder paste, which is adjusted to between 100 and 400 Pa s, forexample. If the viscosity of the solder paste is lower than 100 Pa·s,when the openings are filled with the solder paste, the resin part(rosin) will be pushed out of the openings, and the wettability of thesolder will be impaired. On the other hand, if the viscosity of thesolder paste is over 400 Pa·s, it will be difficult for the solder pasteto flow into the openings.

[0024] It is preferable for the solvent to comprise a combination of afirst solvent having a boiling point lower than the melting point of thesolder powder, and a second solvent having a boiling point higher thanthe melting point of the solder powder.

[0025] With such a combination, when the solder paste is heated, thefirst solvent will vaporize before the solder powder melts, and thesecond solvent will vaporize after the solder powder has begun to melt.The result of this is that the first and second solvents ensure thatthere is enough solvent to adjust the viscosity of the solder paste,while allowing a reduction in the amount of solvent that vaporizes afterthe solder powder has begun to melt. Consequently, less heat is robbedfrom the solder as heat of vaporization during the vaporization of thesolvent, and there is less drop in solder temperature during heating,which minimizes the problem of unmelted solder powder remaining behind.

[0026] Meanwhile, once the solder begins to melt, the second solventbegins to vaporize, but a specific amount of the second solvent remainsfor a certain length of time thereafter. A specific amount of solventneeds to remain when the solder is melted in order to maintain thefluidity of the rosin or other resin component and to keep the activatorfrom being taken away along with the vaporization of the solvent, andthereby allow the activator to fine its way into all the parts of thesolder and act most effectively. This is the role of the second solvent.

[0027] Thus, combining a first solvent with a second solvent ensuresthat the openings in the mask will be properly filled with solder pasteof the desired viscosity, and the activator effectively acts to causethe solder powder particles to fuse together, allowing solder bumps withlittle variance to be formed. As a result, it is possible to form solderbumps more precisely, and it is possible to form solder bumps accuratelyat a fine pitch on electrode pads provided at a fine pitch, as is thecase with semiconductor elements and so forth.

[0028] For this effect to be achieved in the best way, the first solventis preferably one that has a boiling point 5 to 50° C. lower than themelting point of the solder powder, and the second solvent is preferablyone that has a boiling point 5 to 50° C. higher than the melting pointof the solder powder. In other words, if the boiling point of the firstsolvent is too low, the first solvent may evaporate at room temperature,causing the viscosity of the solder paste to rise, but if the boilingpoint is too high, it will be close to the melting point of the solderpowder, making it impossible to sufficiently reduce the amount of heatrobbed by the vaporization of the first solvent when the solder powdermelts. Meanwhile, if the boiling point of the second solvent is toohigh, it will be impossible to sufficiently vaporize the second solventin the course of heating the solder paste, but if this boiling point istoo low, it will be close to the melting point of the solder powder, theactivator will be taken away as the second solvent vaporizes, and theactivator will not adequately fulfill its function.

[0029] The types of first and second solvents used are determined by themelting point of the solder, and mainly by the composition of the solderpowder. Table 1 below gives typical solder powder compositions andsuitable compositions for the first and second solvents. TABLE 1 Solderpowder Composition (melting First solvent Second solvent point/° C.)Name (boiling point/° C.) Name (boiling point/° C.) 63% Sn—Pb ethyleneglycol monoethyl ether ethylene glycol diacetate (183) (135.0) (190.5)n-butyl ether (140.9) propylene glycol (188.2) diethylene glycoldimethyl 2-methyl-2,4-pentanediol ethyl ether (145.0) (197.0) ethyleneglycol monomethyl ethylene glycol (197.7) ether acetate (145.1) ethyleneglycol dibutyl methyl phenyl ether (153.9) ether (203.6) ethylene glycolmonoethyl ether ethylene glycol acetate (156.8) monohexyl ether (208.3)diethylene glycol dimethyl n-butyl phenyl ether ether (159.6) (213.3)diethylene glycol methoxymethoxyethanol (167.5) monoethyl ether acetate(217.4) ethyl phenyl ether (170.1) α-terpenol (218.0) propylene glycolmonobutyl dipropylene glycol (229.2) ether (171.1)1-butoxyethoxypropanol (229.4) ethylene glycol monobutyl etherdiethylene glycol monobutyl (171.2) ether (230.4) Sn-3.5% Ag ethyleneglycol isoamyl ether dipropylene glycol (229.2) (221) (181.0)1-butoxyethoxypropanol (229.4) diethylene glycol diethyl etherdiethylene glycol monobutyl (186.0) ether (230.4) ethylene glycolmonoacetate ethylene glycol monophenyl (187.0) ether (237.0) propyleneglycol (188.2) 1,5-pentanediol (242.5) dipropylene glycol monomethyltripropylene glycol monomethyl ether (190.0) ether (243.0) ethyleneglycol diacetate diethylene glycol (245.0) (190.5) diethylene glycolmonobutyl ethylene glycol monobutyl ether ether acetate (246.8) acetate(191.5) diethylene glycol monoacetate diethylene glycol monomethyl(250.0) ether (194.2) diethylene glycol dibutyl ether diethylene glycolmonoethyl (254.6) ether (195.0) ethylene glycol benzyl ether2-methyl-2,4-pentanediol (256.0) (197.0) ethylene glycol monophenyl3,4-hexylene glycol (197.1) ether acetate (259.7) dipropylene glycolmonoethyl glyceryl monobutyrate (269.0) ether (197.8) ethylene glycoldibutyl ether (203.6) ethylene glycol monohexyl ether (208.3) n-butylphenyl ether (213.3) 5% Sn—Pb glyceryl monobutyrate (269.0) benzylbenzoate (323.0) (315) dimethyl phthalate (283.7) dibutyl phthalate(339.0) diethyl phthalate (302.0) dioctyl phthalate (340.0) ethylabietate (350.0) amyl stearate (360.0)

[0030] It is preferable for the first and second solvents each to becontained in an amount of 2 to 6 wt % in the solder paste in order forthem to fulfill their above-mentioned roles as the first and secondsolvents.

[0031] A thixotropic agent may be admixed to the flux vehicle in orderto impart shape retention properties to the solder paste. Any of avariety of known thixotropic agents can be used, such as hardened castoroil or hydroxystearic acid.

[0032] All of the components used as constituent components of thesolder paste preferably either contain no halogen elements or alkalimetal elements, or contain these in extremely small amounts. This isbecause if halogen elements or alkali metal elements remain after thesolder bumps have been formed, corrosion can cause degradation of thesemiconductor element, or migration can cause shorting between theelectrodes. It is particularly favorable for the halogen element andalkali metal element content in the flux vehicle to be no more than 100ppm.

[0033] In a preferred embodiment, the mask is provided over thesubstrate through the steps of forming a first cover layer over thesubstrate, forming a second cover layer over this first cover layer, andforming the plurality of openings in the first cover layer and thesecond cover layer by exposing these to light in a pattern correspondingto the plurality of electrode pads and developing with an etchant, andthe first cover layer is formed from a material that will be dissolvedby the etchant used to develop the second cover layer, with the etchingof the first cover layer being carried out simultaneously with thedeveloping of the second cover layer.

[0034] With this bump formation method, just the portion of the secondcover layer corresponding to the electrode pads is selectively removedin the developing that follows optical exposure, and first openings thatconstitute the above-mentioned openings are formed in the second coverlayer. Meanwhile., because the first cover layer is formed from amaterial that will be dissolved by the etchant used to develop thesecond cover layer, the first cover layer is also etched at the sametime by the above-mentioned etchant. Here, since the first cover layeris formed underneath the second cover layer, the second cover layer inwhich the first openings are formed functions as an etching mask for thefirst cover layer. Therefore, just the portion of the first cover layercorresponding to the electrode pads and corresponding to the firstopenings formed in the second cover layer is selectively removed to formsecond openings that constitute the above-mentioned openings. Thus, withthe above bump formation method, there is no need for the first coverlayer and the second cover layer to be etched separately in theformation of the openings, which is advantageous in that the work ismore efficient.

[0035] Unless otherwise specified, the term “optical exposure”encompasses irradiation with X-rays, an electron beam, or the like.

[0036] The material used to form the first cover layer should be onethat will be dissolved by the etchant used in the developing of thesecond cover layer, and may be suitably selected as dictated by the typeof etchant being used.

[0037] The material used to form the second cover layer is amacromolecular compound that is photosensitive, or a mixture of aphotosensitive compound and another compound, for example, but can beeither a negative type in which the portion irradiated with light iscured, or a positive type in which the portion irradiated with light isdecomposed. The meaning of the word “photosensitive” here is not limitedto the property of undergoing curing (reaction) or decomposition(reaction) when irradiated with light, and also encompasses the propertyof undergoing curing (reaction) or decomposition (reaction) whenirradiated with an electron beam, X-rays, or the like.

[0038] Examples of materials with which a negative type cover layer canbe formed include polymerizable vinyl group-containing vinyl esters,styrene, acrylic esters, methacrylic esters, and other such monomers, aswell as oligomers of these monomers, unsaturated polyester resins, andurea acrylates, and acrylic monomers and oligomers having polymerizableunsaturated double bonds. Naturally, a negative type cover layer may beformed from just a photosensitive compound, or it may be formed from amixture of a photosensitive compound and another compound, such as anacrylic-, epoxy-, or imide-based macromolecular compound.

[0039] Examples of materials with which a positive type cover layer canbe formed include macromolecular compounds having ether bonds thatreadily undergo photolysis (such as polyethylene oxide, cellulose, andpolyacetal), as well as polyethylene and other macromolecular compoundsthat readily produce radicals under optical irradiation, and mixtures oflow molecular weight compounds that are decomposed by opticalirradiation, such as diazo compounds, with another compound.

[0040] In a preferred embodiment, the first cover layer is formed from amaterial containing a macromolecule that is water-soluble or readilydissolves in an alkaline aqueous solution.

[0041] With this bump formation method, the second cover layer can beremoved at the same time if at least the first cover layer is dissolvedby water or an aqueous solution such as an alkaline aqueous solution.Specifically, if the second cover layer is formed from a materialcontaining a macromolecule that is water-soluble or readily dissolves inan alkaline aqueous solution just as is the first cover layer, then thesecond cover layer can also be dissolved away at the same time by wateror an aqueous solution such as an alkaline aqueous solution. On theother hand, if the second cover layer is formed from a material thatcontains as its main component a macromolecule has poor solubility inwater or in an alkaline aqueous solution, then just the first coverlayer can be dissolved. Since the first cover layer is formed underneaththe second cover layer, once the first cover layer is dissolved, thesecond cover layer will no longer be attached to the substrate. In thisstate, the second cover layer can be easily removed in the form of afilm, even if the second cover layer itself is not dissolved. Sincethere is no need to dissolve the second cover layer in this case, anadvantage is that less water or aqueous solution such as an alkalineaqueous solution is used. Therefore, in this respect it is preferable toform the second cover layer from a material whose main component is amacromolecular that has poor solubility in water or in an alkalineaqueous solution.

[0042] When the second cover layer has poor solubility in water or in analkaline aqueous solution, it is preferable for the second cover layerto contain a macromolecule based on an acrylic (such as an acrylicester), an epoxy (such as a bisphenol A type), or an imide (such as abismaleimide type of polyimide). Naturally, a combination of thesemacromolecules may also be used.

[0043] The macromolecule that is water-soluble or readily dissolves inan alkaline aqueous solution and is contained in the first cover layercan be a natural macromolecule such as animal-derived gelatin orvegetable-derived starch, a semi-synthetic macromolecule such as astarch derivative or a cellulose derivative, as well as various othermacromolecules. Homopolymers (straight polymers) and copolymers can bothbe used as synthetic macromolecules. Examples of homopolymers includepolyvinyl alcohol, polyvinyl pyrrolidone, and other vinyl-basedpolymers, polyacrylamide, polyacrylic acid, and other acrylic polymers,and polyethylene oxide. Examples of copolymers include random copolymerssuch as a partially saponitied polyvinyl acetate, block copolymers suchas poly(styrene-ethylene oxide), and graft copolymers such aspoly(ethylene-vinyl alcohol)-g-(ethylene oxide).

[0044] In a preferred embodiment, the plurality of electrode pads aredivided into a plurality of groups, and the mask is formed through thesteps of forming a cover layer so as to cover the plurality of electrodepads, and forming the plurality of openings in this cover layer in apattern corresponding to the plurality of electrode pads, with thevolume of these openings being different for each group.

[0045] With this bump formation method, the amount of solder pastefilling the various openings is different for each group of electrodepads. Accordingly, it is possible for the bums formed on the electrodepads to be different sizes for each group.

[0046] For example, if first openings formed corresponding to thevarious members of a first electrode pad group out of a plurality ofgroups are larger in volume than second openings formed corresponding tothe various members of a second electrode pad group out of a pluralityof groups, then the amount of solder paste filling the first openingswill be greater than the amount of solder paste filling the secondopenings. Accordingly, when the bumps are finally formed, those bumpsformed on the first electrode pads will be larger than the bumps formedon the second electrode pads.

[0047] In a preferred embodiment, the plurality of electrode pads aredivided into a group comprising a plurality of first electrode pads anda group comprising a plurality of second electrode pads, each of thefirst electrode pads being formed in a surface area smaller than each ofthe second electrode pads, and the plurality of openings include aplurality of first openings formed in a pattern corresponding to theplurality of first electrode pads, and a plurality of second openingseach smaller in volume than each of the first openings and formed in apattern corresponding to the plurality of second electrode pads.

[0048] With this bump formation method, if the thickness of the coverlayer is uniform under conditions in which no molten solder is incontact with the inner walls of the openings when the solder is melted,for instance, then the solder bumps will be taller when formed inopenings of greater volume. In other words, the larger is an opening,the greater is the amount of solder paste that fills it, so the bumpformed on that electrode pad will be taller. If an electrode pad issmall, then there will be less contact surface area between theelectrode pad and the bump, and the bump will be closer to spherical inshape, so the height of the bumps can be varied in this respect as well.

[0049] Thus, if openings of different size are formed in the coverlayer, and the surface area of the electrode pads is also different,then a plurality of bump groups of varying distance from the electrodepads to the bump tops can be formed simultaneously and in the sa mestep.

[0050] The above description is of an example in which two types of bumpwith different heights are formed, but of course the present inventioncan also be applied to when three or more types of bump of differentheights are formed. For instance, in addition to first and secondopenings of different open volume, third openings with yet a differentvolume may be provided, and of course fourth or further openings mayalso be provided.

[0051] The cover layer is formed, for example, by coating with a moltenresin, or laying down a resin film. However, forming the cover layer bylaying down a resin film is advantageous, not only because the step offorming the cover layer is easier, but also because it is possible toform a cover layer of uniform thickness with ease.

[0052] The cover layer can be made up of a highly insulating resin basedon a resin such as polymethyl methacrylate, polyacrylate, or polymethylisopropenyl ketone, and is preferably made up of a photosensitivematerial containing a photopolymerizable monomer such as apolyfunctional acrylate.

[0053] In a preferred embodiment, wherein the plurality of electrodepads include a plurality of first electrode pads and a plurality ofsecond electrode pads, and the plurality of openings include a pluralityof first openings, a plurality of second openings, and a plurality ofthird openings, and the mask is formed through the steps of forming afirst cover layer by covering the plurality of first electrode pads andexposing the plurality of second electrode pads, forming the pluralityof first openings in this first cover layer in a pattern correspondingto the plurality of first electrode pads, forming a second cover layerso as to cover the first cover layer and the plurality of secondelectrode pads, forming the plurality of second openings in the secondcover layer in a pattern corresponding to the plurality of secondelectrode pads, and forming the plurality of third openings in a patterncorresponding to the plurality of first openings.

[0054] With this bump formation method, a mask is constituted by thefirst cover layer and the second cover layer in the region where thefirst electrode pads are formed, and the mask is constituted by only thesecond cover layer in the region where the second electrode pads areformed. The first and third openings are provided over the firstelectrode pads, and the second openings are formed over the secondelectrode pads. Since the second and third openings are both formed inthe second cover layer, if the thickness of the second cover layer isuniform, the depth of these openings will be the same. Accordingly, theopenings formed over the first electrode pads are deeper than thoseformed over the second electrode pads by the depth of the first openings(the thickness of the first cover layer). Therefore, when theabove-mentioned mask is used, the amount of solder paste resting on thefirst electrode pads will be greater than that resting on the secondelectrode pads, and the solder bumps formed thereon will also be taller.As a result, with this bump formation method, it is possible to form aplurality of bump groups with significantly varying distances from thesubstrate surface to the bump tops.

[0055] The third openings are preferably formed larger than the secondopenings. If they are, then the amount of solder paste resting on thefirst electrode pads will be larger than the amount of solder pasteresting on the second electrode pads, and as a result the height of thebumps formed on the first electrode pads can be significantly differentfrom the height of the bumps formed on the second electrode pads.

[0056] In a preferred embodiment, the third openings are formed with alarger open surface area than the first openings, and there is furtherincluded a step of selectively removing just the second cover layer,with the first cover layer left on the substrate.

[0057] With this bump formation method, the first cover layer, which hasfirst openings with a smaller open surface area than the third openings,remains after the bumps have been formed, so bumps in which a sphericalportion, example, protrudes from the surface of the first cover layerare formed over the first electrode pads such that they are raised up tothe remaining first cover layer. Meanwhile, spherical bumps, forexample, are formed directly on the second electrode pads. Accordingly,a height difference can be achieved between the bumps over the firstelectrode pads and the bumps over the second electrode pads.

[0058] The first and second cover layers are formed, for example, bycoating with a molten resin, or laying down a resin film. However,forming the cover layers by laying down a resin film is advantageous,not only because the step of forming the cover layer is easier, but alsobecause it is possible to form a cover layer of uniform thickness withease.

[0059] The first cover layer can be made up of a highly insulating resinbased on a resin such as epoxyacrylate, epoxy, and polyimide.

[0060] The second cover layer can be made up of a highly insulatingresin based on a resin such as polymethyl methacrylate, polyacrylate, orpolymethyl isopropenyl ketone, and is preferably made up of aphotosensitive material containing a photopolymerizable monomer such asa polyfunctional acrylate. The first cover layer must be a material thatexhibits chemical properties different from those of the second resinfilm so that it will not be etched by the etchant when the second andthird openings are formed in the second resin film. For example, thefirst resin film can be made up of a material such as epoxyacrylate,epoxy, and polyimide. If it is, not only will the step of forming theresin film be easier, but it will also be possible to form a resin filmof uniform thickness with ease.

[0061] In a preferred embodiment, the filling of the openings withsolder paste is carried out through the steps of holding the substrateon a substrate support, providing squeegeeing helper means for lesseningthe difference between the height position of the mask and the heightposition of the periphery of the substrate, readying solder paste on themask or the squeegeeing helper means, and moving a squeegee to push thesolder paste down into the openings.

[0062] The filling of the openings with the solder paste need onlycomprise the various steps listed above, and does not necessarily haveto follow the above order. For instance, the openings may be formedafter the formation of the cover layers on the substrate, and theopenings then with the solder paste while the substrate is held on asubstrate support.

[0063] With this bump formation method, the provision of the squeegeeinghelper means lessens the difference between the height position of themask and the height position of the periphery of the substrate.Accordingly, the squeegee can be moved not only over the cover layer,but also over the squeegeeing helper means. In other words, not only thesolder paste on the substrate, but also the solder paste on thesqueegeeing helper means can be moved at the same time and used to fillin the openings. This means that the various openings can be filled withsolder paste easily and reliably even when the bumps are being formed ona substrate with uneven width dimensions (such as a silicon wafer).

[0064] As shown in FIG. 18a, when bumps were formed on electrode pads 15a of a disk-shaped substrate 15 such as a silicon wafer, the followingproblems were encountered in the filling of the openings 16 a in a mask16 with a solder paste P. The filling of the openings 16 a with thesolder paste P was carried out by readying the solder paste P along aspecific edge 16A of the mask 16 and moving a squeegee S to the edge 16Bon the opposite side. Here, if we look at the movement path of thesqueegee S, we see that, as shown in FIG. 18b, the size of the substrate15 (mask 16) in the direction perpendicular to the movement direction ofthe squeegee S increases along with the movement of the squeegee S atfirst, but decreases after passing the widest portion. Therefore, if thesolder paste P is readied near the starting point of the squeegee S, thesolder paste P can only be moved in a width roughly corresponding to thelength of the readied solder paste P. This makes it difficult toproperly fill the openings 16 a′ formed along the edge of theabove-mentioned widest portion with a sufficient amount of the solderpaste P′. Also, if the openings 16 a are formed right up to the edge ofthe mask 16, then when the squeegee S is moved up to the opposite edge16B of the mask 16 in order to fill these openings 16 a with the solderpaste, the solder paste P will end up being scraped off the mask 16.Accordingly, this solder paste P cannot be moved back to the startingportion of the squeegee S by using the squeegee S, which is a problem inthat the solder paste is not utilized effectively.

[0065] In contrast, with the above-mentioned solder bump formationmethod, the periphery of the substrate is surrounded by the squeegeeinghelper means, so if the squeegeeing helper means is taken into account,the size in the direction perpendicular to the movement direction of thesqueegee can be made larger than the widest section of the substrate,and the distance that the squeegee moves can also be made larger thanthe substrate.

[0066] Therefore, if solder paste is readied on the cover layer or onthe squeegeeing helper means so as to correspond to, or be longer than,the widest section of the substrate, and this is moved by the squeegee,then even those openings formed at the widest section of the substrate,or in the vicinity thereof, can be properly filled with solder paste.Also, if the distance the squeegee moves during filling can be madelarger, then the solder paste can be moved back on the squeegeeinghelper means even when the squeegee has reached the edge of thesubstrate, so this solder paste can be reused.

[0067] For this effect to be achieved in the best way, it is preferablefor the squeegeeing helper means and the cover layer to be in the sameor approximately the same plane, but as long as the movement of thesqueegee is not hindered, there may be some difference in the height ofthese.

[0068] The squeegeeing helper means is provided by forming a resin layerso as to surround the periphery of the substrate, or by disposing aplate or the like having an opening corresponding to the shape of thesubstrate so as to surround the periphery of the substrate. Thesqueegeeing helper means may have an opening through which all of theopenings provided to the cover layer can be exposed, and as long as themovement of the squeegee is not hindered, the squeegeeing helper meansmay be provided so that it covers the cover layer, and the surface ofthe squeegeeing helper means is higher than the surface of the coverlayer. Naturally, to the extent that the object of the present inventioncan still be achieved, the squeegeeing helper means does not necessarilyhave to be provided so as to surround the entire periphery of thesubstrate, and need only be provided to the required region of thesubstrate periphery. Also, the squeegeeing helper means does notnecessarily have to be provided as a single integrated member orelement, and a plurality of members or elements may be combined to makeup the squeegeeing helper means.

[0069] However, the resin layer cannot be reused after it has beenremoved following bump formation, but a plate can be used over and over,so in this respect it is preferable to provide the squeegeeing helpermeans by disposing a plate. Also, if the squeegeeing helper meanscomprises a plate, any excess solder paste that did not fill theopenings in the cover layer can be moved back on the squeegeeing helpermeans, and this solder paste that has been moved back to the squeegeeinghelper means can be reused in the formation of bumps on the nextsubstrate.

[0070] When the squeegeeing helper means is formed from a resin layer,it is preferable to form the resin layer from a material that dissolvesin the same etchant as the cover layer, for example, in order to removethe resin layer at the same time in the removal of the cover layer.

[0071] Meanwhile, it is preferable for the substrate support to have arecess capable of accommodating at least part of the substrate. If itdoes, then movement of the substrate with respect to the substratesupport can be restricted even when the squeegee is moved over thesubstrate in order to fill the openings with the solder paste. Since thesubstrate can be set in place on the substrate support just by placingthe substrate in the recess, and does not need to be fixed to thesubstrate support with an adhesive or the like, this is advantageous interms of both cost and work efficiency.

[0072] The bump formation method provided by the second aspect of thepresent invention is a method for forming bumps on a substrate providedwith a plurality of electrode pads, comprising the steps of forming afirst cover layer over the substrate, forming a second cover layer overthe first cover layer, forming a plurality of openings corresponding tothe plurality of electrode pads in the first cover layer and the secondcover layer by exposing these to light and developing with an etchant,filling each of the openings with metal, and heating the metal tointegrate it with the electrode pads, wherein the first cover layer isformed from a material that will be dissolved by the etchant used todevelop the second cover layer, and the first cover layer is etched toform the plurality of openings simultaneously with the developing of thesecond cover layer.

[0073] With this bump formation method, at the same time that the firstopenings constituting the above-mentioned openings are formed in thesecond cover layer by the etching of the second cover layer, the secondopenings constituting the above-mentioned openings are also formed inthe first cover layer using the second cover layer as a mask. Therefore,openings can be formed substantially in the second cover layer byetching without separately etching the cover layers in the formation ofthe openings.

[0074] In a preferred embodiment, the first cover layer is formed from amaterial containing a macromolecule that is water-soluble or readilydissolves in an alkaline aqueous solution.

[0075] With this bump formation method, the second cover layer dissolvesat the same time if at least the first cover layer is dissolved by wateror an aqueous solution such as an alkaline aqueous solution, or thesecond cover layer is separated from the substrate, so the entire maskcan be removed.

[0076] The bump formation method provided by the third aspect of thepresent invention is a method for forming bumps on a substrate providedwith a plurality of electrode pads divided into a plurality of groups,comprising the steps of forming a mask having a plurality of openingscorresponding to the plurality of electrode pads such that the size isdifferent for each group, filling the openings with solder paste,forming bumps from the solder paste by heat treatment, and removing thecover layer from the substrate.

[0077] With this bump formation method, the amount of solder pastefilling the various openings is different for each group of electrodepads. Accordingly, it is possible for the bums formed on the electrodepads to be different sizes, and for the height of the bumps to bedifferent for each group.

[0078] One way to make the height of the bumps different for each groupis to divide the plurality of into a group of first electrode pad groupsand a group of second electrode pads with a larger surface area, andmake the volume of the first openings formed in a pattern correspondingto the first electrode pads smaller than the volume of the secondopenings corresponding to the second electrode pads.

[0079] The larger the amount of solder paste on the electrode pads, thelarger the bumps will be formed, and if the thickness of the cover layeris uniform and the molten solder does not touch the inner walls of theopenings, then the smaller is the surface area of the electrode pads,the taller the bumps will be. Accordingly, if the relationship betweenthe openings and the electrode pads is as above, then bumps of differentheights can be formed more reliably.

[0080] The cover layer is formed, for example, by coating with a moltenresin, or laying down a resin film. Examples of the component that makesup this cover layer include polymethyl methacrylate, polyacrylate, orpolymethyl isopropenyl ketone. These components may be used singly or incombinations of two or more types.

[0081] The bump formation method provided by the fourth aspect of thepresent invention is a method for forming bumps on a substrate providedwith a plurality of first electrode pads and a plurality of secondelectrode pads, comprising the steps of forming a first cover layer in astate in which the plurality of first electrode pads are covered and theplurality of second electrode pads are exposed, forming a plurality offirst openings in the first cover layer in a pattern corresponding tothe plurality of first electrode pads, forming a second cover layer soas to cover the first cover layer and the plurality of second electrodepads, forming a plurality of second openings in the second cover layerin a pattern corresponding to the plurality of second electrode pads,and forming a plurality of third openings in a pattern corresponding tothe plurality of first openings, filling the first openings, secondopenings, and third openings with solder paste, forming bumps from thesolder paste by heat treatment, and removing the second cover layer.

[0082] With this bump formation method, the thickness of the mask isdifferent in the region in which the first electrode pads are formed andin the region in which the second electrode pads are formed. Therefore,the height is different between the bumps formed on the first electrodepads and the bumps formed on the second electrode pads.

[0083] It is preferable for the surface area of the third openings to bemade larger than that of the second openings in order to make theamounts of solder paste filling the insides of the second and thirdopenings markedly different and to achieve a good difference in heightbetween the bumps on the first electrode pads and those on the secondelectrode pads.

[0084] Also, the third openings may be formed with a larger open surfacearea than the first openings, and just the second cover layer may beselectively removed, with the first cover layer left on the substrate.

[0085] If this is done, the first cover layer remains after the bumpshave been formed, so bumps in which a spherical portion, example,protrudes from the surface of the first cover layer are formed over thefirst electrode pads such that they are raised up to the remaining firstcover layer. Meanwhile, spherical bumps, for example, are formeddirectly on the second electrode pads. Accordingly, a good difference inheight can be obtained between the bumps over the first electrode padsand the bumps over the second electrode pads.

[0086] The first and second cover layers are formed, for example, bycoating the substrate with a molten resin, or laying a resin film overthe substrate.

[0087] The first cover layer can be made up of a highly insulating resinbased on a resin such as epoxyacrylate, epoxy, and polyimide.

[0088] The second cover layer can be made up of a highly insulatingresin based on a resin such as polymethyl methacrylate, polyacrylate, orpolymethyl isopropenyl ketone.

[0089] The bump formation method provided by the fifth aspect of thepresent invention is a method for forming bumps on a substrate providedwith a plurality of electrode pads, comprising the steps of holding thesubstrate on a substrate support, forming a cover layer so as to coverat least the substrate, forming a plurality of openings in the coverlayer in a pattern corresponding to the plurality of electrode pads,providing squeegeeing helper means for lessening the difference betweenthe height position of the cover layer on the substrate and the heightposition of the periphery of the substrate, readying a metal paste(including solder paste) or metal powder on the cover layer or thesqueegeeing helper means, moving a squeegee to push the metal paste ormetal powder down into the openings, heating, melting, and solidifyingthe metal paste or metal powder to integrate it on the electrode pads,and taking away the squeegeeing helper means.

[0090] This bump formation method need only comprise the various stepslisted above, and does not necessarily have to follow the above order.For instance, the openings may be formed after the formation of thecover layer on the substrate, and the openings then filled with thesolder paste while the substrate is held on a substrate support.

[0091] With this bump formation method, the provision of the squeegeeinghelper means lessens the difference between the height position of themask and the height position of the periphery of the substrate.Accordingly, the metal powder or metal paste can be moved not only overthe cover layer, but also by utilizing the squeegeeing helper means,allowing the individual openings to be reliably filled with metalpowder, etc., even when the bumps are formed on a substrate whose widthis not even.

[0092] For this effect to be achieved in the best way, it is preferablefor the squeegeeing helper means and the cover layer to be in the sameor approximately the same plane, but as long as the movement of thesqueegee is not hindered, there may be some difference in the height ofthese.

[0093] The squeegeeing helper means is provided by forming a resin layerso as to surround the periphery of the substrate, or by disposing aplate or the like having an opening corresponding to the shape of thesubstrate so as to surround the periphery of the substrate. Thesqueegeeing helper means may have an opening through which all of theopenings provided to the cover layer can be exposed, and as long as themovement of the squeegee is not hindered, the squeegeeing helper meansmay be provided so that it covers the cover layer, and the surface ofthe squeegeeing helper means is higher than the surface of the coverlayer.

[0094] Meanwhile, it is preferable for the substrate support to have arecess capable of accommodating at least part of the substrate. If itdoes, then movement of the substrate with respect to the substratesupport can be restricted, allowing the squeegee to move more smoothly.

[0095] In all of the first to fifth aspects of the present inventiondiscussed above, it is preferable if there is further provided a step ofapplying flux to the bumps formed from heat treated solder paste, andperforming a heat treatment again to adjust the shape of the bumps.

[0096] A flux containing Polypale and hexylene glycol is used, forexample.

[0097] In all of the first to fifth aspects of the present inventiondiscussed above, it is preferable if the open surface area of theopenings is no more than 25 times the surface area of the correspondingelectrode pads. If it is, the molten solder can be gathered morereliably on the electrode pads when the solder is melted, allowing thesolder to be formed in good spherical shapes.

[0098] The sixth aspect of the present invention provides an electroniccomponent, comprising a substrate, a plurality of first electrode padsand a plurality of second electrode pads formed on the same surface ofthis substrate, a plurality of first bumps formed in a patterncorresponding to the plurality of first electrode pads, and a pluralityof second bumps formed in a pattern corresponding to the plurality ofsecond electrode pads, wherein the surface area of each of the firstelectrode pads is smaller than the surface area of each of the secondelectrode pads, and the top of each of the first bumps is located higherthan the top of each of the second bumps.

[0099] The seventh aspect of the present invention provides anelectronic component, comprising a substrate, a plurality of firstelectrode pads and a plurality of second electrode pads formed on thesame surface of this substrate, a cover layer formed in the region ofthe substrate where the plurality of first electrode pads are formed andhaving a plurality of openings corresponding to the plurality of firstelectrode pads, a plurality of first bumps provided in a patterncorresponding to the plurality of first electrode pads, with sphericalportions protruding from the cover layer, and a plurality of secondbumps provided in a pattern corresponding to the plurality of secondelectrode pads, with spherical portions formed directly on thecorresponding second electrode pads, wherein the top of each of thefirst bumps is located higher than the top of each of the second bumps.

[0100] The cover layer can be made from a highly insulating resin basedon an epoxyacrylate, epoxy, polyimide, or other such resin.

[0101] A preferred embodiment of the electronic components discussed inthe above-mentioned sixth and seventh aspects of the present inventionis an electronic component further comprising a mounting object, whereinthis mounting object is placed on the substrate with the plurality ofsecond bumps therebetween, and the top of each of the first bumps islocated at a height of at least 1.2 times the height location of the topof the mounting object.

[0102] With this structure, an additional mounting object can be placedon the substrate via the first bumps in a state in which the originalmounting object is interposed between the additional mounting object andthe substrate, or the substrate can be mounted on another substrate viathe first bumps. Employing this structure affords higher mountingefficiency in the mounting of an electronic component on the substrate,and allows for a more compact electronic component consisting of aplurality of semiconductor chips or the like.

[0103] An electronic component having two kinds of bumps of differentsize was described in the sixth and seventh aspects of the presentinvention, while a method for forming two kinds of bumps of differentsize as needed was described in the first through fourth aspects of thepresent invention. However, when three or more kinds of bump ofdifferent size are to be formed, the present invention can be appliedwhenever any two bumps are of different size, and is not necessarilylimited to when two kinds of bump of different size are formed.

[0104] The eighth aspect of the present invention provides a solderpaste containing a solder powder and a solvent, wherein the solventcontains a first solvent having a boiling point lower than the meltingpoint of the solder powder, and a second solvent having a boiling pointhigher than the melting point of the solder powder.

[0105] This solder paste can be used favorably in the formation ofsecond bumps. In a preferred embodiment, the same solder paste as thatdiscussed in the eighth aspect of the present invention can also be usedin the first aspect of the present invention. Therefore, when the solderpaste of the eighth aspect of the present invention is used to formsecond bumps, the same effects will be realized as when the solder pasteof the first aspect of the present invention was used.

[0106] Specifically, since the first solvent has already vaporized inthe melting of the solder, less heat is robbed from the solder throughheat of vaporization of the solvent after the solder begins to melt, theeffect being an amelioration of the problem of unmelted solder powderbeing left behind. Because a specific amount of the second solventremains even after the solder has melted, the fluidity of the rosin orother resin component is maintained, the activator is not carried awayas the solvent is vaporized, and the activator is able to get into allparts of the solder, allowing it to act more effectively. As a result,it is possible to form solder bumps with a good spherical shape and novariance in size.

[0107] For this effect to be obtained in an even better way, it ispreferable for the first solvent to have a melting point 5 to 50° C.lower than the melting point of the solder powder, and for the secondsolvent to have a melting point 5 to 50° C. higher than the meltingpoint of the solder powder. For the same reason, it is preferable forthe first solvent to be contained in the solder paste in an amount of 2to 6 wt %, and the second solvent in an amount of 2 to 6 wt %.

[0108] The types of first and second solvent to be used will bedetermined by the type (melting point) of the solder powder being used,and in the eighth aspect of the present invention, it is againpreferable to use those listed as examples for the first aspect of thepresent invention (see Table 1).

BRIEF DESCRIPTION OF THE DRAWINGS

[0109]FIGS. 1a to 1 e are cross sections illustrating the steps offorming a mask in the bump formation method pertaining to a firstembodiment of the present invention;

[0110]FIGS. 2a and 2 b are a cross section and a plan view illustratingthe step of providing the squeegeeing helper means around the substrate;

[0111]FIGS. 3a and 3 b are a cross section and a plan view illustratingthe printing step;

[0112]FIGS. 4a and 4 b are a cross section and a plan view illustratingthe printing step;

[0113]FIGS. 5a to 5 d are cross sections illustrating the final bumpformation steps;

[0114]FIGS. 6a to 6 f are cross sections illustrating the bump formationmethod pertaining to a second embodiment of the present invention;

[0115]FIG. 7a is a cross section giving an enlarged detail view of FIG.6a, and FIG. 7b is a cross section giving an enlarged detail view ofFIG. 6b;

[0116]FIG. 8 is a perspective view showing an example of an electronicelement obtained through the bump formation method pertaining to thesecond embodiment of the present invention;

[0117]FIG. 9 is a perspective view showing the state in which sub-chipsare mounted on an electronic element (main chip) in FIG. 8;

[0118]FIGS. 10a and 10 b are perspective views illustrating the steps ofmounting the sub-chips on the electronic element (main chip) in FIG. 8;

[0119]FIG. 11 is a perspective view showing the state in which the mainchip in the state in FIG. 9 is mounted on a rewired substrate;

[0120]FIGS. 13a to 13 h are cross sections illustrating the bumpformation method pertaining to a third embodiment of the presentinvention;

[0121]FIGS. 14a to 14 d are cross sections showing the state in which asub-chip is mounted on the electronic element (main chip) in FIG. 13,and this assembly is mounted on a rewired substrate;

[0122]FIG. 15 is a cross section of the semiconductor chip in a fourthembodiment pertaining to the present invention; ′FIG. 16 is a crosssection showing the state in which sub-chips are mounted on thesemiconductor chip in FIG. 15;

[0123]FIG. 17 is a cross section showing the state in which asemiconductor chip in the state in FIG. 16 is placed on a rewiredsubstrate; and

[0124]FIGS. 18a and 18 b are a cross section and a plan viewillustrating the printing step in a conventional bump formation method.

DESCRIPTION OF THE PREFFERED EMBODIMENTS

[0125] Preferred embodiments of the present invention will now bedescribed in detail through reference to the drawings.

[0126] First, the bump formation method pertaining to the firstembodiment of the present invention will be described in specific termsthrough reference to FIGS. 1 to 5. In this first embodiment we willdescribe the method for forming bumps on a circular substrate.

[0127] The bump formation method pertaining to the first embodiment isbroadly divided into a step in which a mask is formed, a step in which asqueegeeing helper means is provided, a printing step, and a step inwhich the bumps are finally formed.

[0128] The step of forming a mask involves positioning a substrate 1shown in FIG. 1a, forming a first cover layer 21 shown in FIG. 1b,forming a second cover layer 22 shown in FIG. 1c, and forming openings23 shown in FIGS. 1d and 1 e.

[0129] The positioning of the substrate 1 is performed by putting thesubstrate 1 inside a recess 40 in a substrate support 4 as shown in FIG.1a. The recess 40 has an open surface area corresponding to the planview surface area of the substrate 1, but its depth is less than thethickness of the substrate 1. Accordingly, only the lower part of thesubstrate 1 fits into and is positioned in the recess 40, whichrestricts the movement of the substrate 1.

[0130] When the substrate 1 is positioned in this way, there, is no needto use an adhesive or the like to fix the substrate 1 to the substratesupport 4, which makes the work easier. A plurality of electrode pads 10are provided to the substrate 1.

[0131] The formation of the first cover layer 21 is performed by coatingthe substrate 1 with a solution of a polyacrylic acid, polyvinylalcohol, or other such macromolecule with high water solubility by aknow process such as spin coating or screen printing (see FIG. 1b). Thethickness of the first cover layer 21 is set to about 0.01 to 10 μm, forexample.

[0132] The formation of the second cover layer 22 is performed bysubjecting a resin film containing a highly photopolymerizable orphotodegradable (photosensitive) material to hot press bonding over thefirst cover layer 21 (see FIG. 1c). This resin film is a negative typein which the portion irradiated with light is polymerized, for example;specifically, it comprises a photopolymerization initiator mixed with anacrylic ester or methacrylic ester. The thickness of the second coverlayer 22 is set according to the height of the bumps to be formed andthe thickness of the first cover layer 21, but when the height of thebumps is 75 μm, for example, this thickness is about 20 to 60 μm.

[0133] The formation of the openings 23 is performed by forming secondopenings 22 a corresponding to the second cover layer 22 and, at thesame time, forming first openings 21 a corresponding to the first coverlayer 21, as shown in FIGS. 1d and 1 e.

[0134] In the formation of the second openings 22 a, when the secondcover layer 22 is formed using a negative type resin, for example, firstthe portion corresponding to the electrode pads 10 is irradiated withlight in a state in which a photomask with an opaque component 50 is inplace, as shown in FIG. 1d. Then, as shown in FIG. 1e, the photomask 5is taken off, and the second openings 22 a are formed by removing theportion not irradiated with light (non-polymerized component) using anetchant such as a tetramethylammonium hydroxide aqueous solution. Here,the second cover layer 22 in which the second openings 22 a are formedfunctions as a mask for the first cover layer 21. If the first coverlayer 21 is formed from a highly water-soluble macromolecular material,then the first cover layer 21 will also be selectively removed only inthe portion corresponding to the electrode pads 10, forming the firstopenings 21 a. The openings 23 (21 a and 22 a) are thus formed in thefirst and second cover layers 21 and 22, and a mask 2 is formed suchthat it covers the substrate 1 and leaves the electrode pads 10 exposed.

[0135] The open surface area of the openings 23 is preferably no morethan 25 times the surface area of the electrode pads 10.

[0136] The step of forming the squeegeeing helper means is carried outby disposing a rectangular frame-shaped plate 6 with an opening 60formed in its middle, such that it surrounds the substrate 1 as shown inFIGS. 2a and 2 b. The plate 6 is entirely formed from a resin such asTeflon or a metal such as stainless steel. The opening 60 in the plate 6has an open surface area substantially corresponding to the plan viewsurface area of the substrate 1, and the thickness of the plate 6substantially corresponds to the difference between the height locationof the second cover layer 22 on the substrate 1 and the height locationof the second cover layer 22 around the substrate 1. Accordingly, withthe plate 6 in place, the height location of the second cover layer 22around the substrate 1 is the same or substantially the same as theheight location of the second cover layer 22 on the substrate 1. As aresult, the squeegee S, the solder paste P, etc., can be moved not onlyover the second cover layer 22 on the substrate 1, but also over thesqueegeeing helper means 6 (see FIGS. 3 and 4).

[0137] Naturally, to the extent that movement of the squeegee S is notlost, there maybe a difference in the height locations of thesqueegeeing helper means 6 and the second cover layer 22. In this case,the height location of the second cover layer 22 may be about 200 μm,for example, lower than the squeegeeing helper means 6.

[0138] The printing step is carried out by moving the solder paste Pwith the squeegee S as shown in FIGS. 3a, 3 b, 4 a, and 4 b so as tofill the openings 23 with the solder paste P.

[0139] First, as shown in FIGS. 3a and 3 b, solder paste P is readied onthe second cover layer 22 and the squeegeeing helper means 6 around theedges of the squeegeeing helper means 6. Here, the length L of theregion in which the solder paste P is readied is preferably longer thanthe maximum width W of the substrate 1. In this embodiment, thesqueegeeing helper means 6 is provided so as to be the same orsubstantially the same height as the second cover layer 22, so thesqueegeeing helper means 6 is also utilized to ready the solder paste P.Doing so allows the solder paste P to be readied so as to be longer thanthe maximum width W of the substrate 1 even when the width of thesubstrate 1 is not consistent. In this case, the squeegee S is longerthan the maximum width W of the substrate 1.

[0140] The solder paste P used in this printing step is one thatcontains solder powder and a flux vehicle, and preferably has aviscosity of 100 to 400 Pa·s.

[0141] Tin, lead, bismuth, zinc, copper, cadmium, antimony, and othersuch components can be used for the solder, with typical examplesincluding 63% Sn—Pb, Sn-3.5% Ag, and 5% Sn—Pb.

[0142] The solder powder that is used contains no more than 10 wt %particles whose diameter is greater than the thickness of the mask 2 andno more than 1.5 times this thickness, for example. Preferably, thesolder powder contains no more than 10 wt % particles with a diameter of40% or more of the open diameter of the openings 23, and even morepreferably, contains at least 30 wt % particles whose diameter is 40 to100% of the thickness of the mask 2.

[0143] The flux vehicle contains rosin, an activator, and a solvent, andpreferably the combined content of halogen elements and alkali metalelements is no more than 100 ppm.

[0144] The rosin can be polymerized rosin, hydrogenated rosin,esterified rosin, or the like. The amount in which the rosin iscontained in the solder paste P is 2 to 6 wt %, for example.

[0145] The activator can be sebacic acid, succinic acid, adipic acid,glutaric acid, triethanolamine, monoethanolamine, or another organicacid or organic amine. The amount in which the activator is used in thesolder paste P is 0.01 to 2 wt %, for example.

[0146] It is preferable for the solvent to include a first solventhaving a boiling point lower than the melting point of the solderpowder, and a second solvent having a boiling point higher than themelting point of the solder powder. Even more preferably, the firstsolvent has a boiling point 5 to 50° C. lower than the melting point ofthe solder powder, and the second solvent has a boiling point 5 to 50°C. higher than the melting point of the solder powder. The amounts inwhich the first and second solvents are contained in the solder paste Pare each about 2 to 6 wt %.

[0147] Next, as shown in FIGS. 4a and 4 b, the squeegee S is positionedso that it covers the solder paste P, and this squeegee S is moved sothat it touches the tops of the squeegeeing helper means 6 and thesecond cover layer 22. In this process, because the length of thesqueegee S and the length L of the region in which the solder paste P isreadied are greater than the maximum width W of the substrate 1, thesqueegee S and the solder paste P move over all of the openings 23, andthe solder paste P′ drops into all of the openings 23. The squeegee Smay also be moved opposite to its previous movement path in an effort toimprove the filling of the openings 23 with the solder paste P.

[0148] The solder paste P used in this embodiment has a small proportionof solder powder particles with relatively large diameter as compared tothe thickness of the mask 2. Consequently, even if the squeegee S ismoved back and forth over the mask 2, there is little danger that thesolder powder that has filled the openings 23 will be scraped back outagain by the squeegee S.

[0149] The step of finally forming the bumps is carried out by firsttaking away the squeegee S and the squeegeeing helper means 6 as shownin FIG. 5a, then heat treating the solder paste and removing the mask 2.

[0150] In taking away the squeegeeing helper means 6, if the excesssolder paste P that has not filled the openings 23 on the squeegeeinghelper means 6 is moved back first, then this excess solder paste P canbe removed simultaneously with the squeegeeing helper means 6.Furthermore, when the squeegeeing helper means 6 onto which the solderpaste P has been moved is used in the printing of the solder paste Ponto the next substrate 1, this solder paste P can be reused.

[0151] The heat treatment of the solder paste P is carried out, forexample, by sending the printed substrate 1 into a heating furnace afterthe squeegee S and the squeegeeing helper means 6 have been removed. Thetemperature in the heating furnace is determined by the type of solderpaste being used (and particularly the solder components), but isroughly 250 to 450° C. This heat treatment melts the solder paste p butthe molten solder forms a ball due to its surface tension. When thismolten solder is cooled and solidified, spherical bumps B are integratedwith the electrode pads 10 as shown in FIG. 5b.

[0152] In this embodiment the solder paste P is one that contains afirst solvent having a boiling point lower than the melting point of thesolder powder, and a second solvent having a boiling point higher thanthe melting point of the solder powder. Accordingly, since the firstsolvent has already vaporized in the melting of the solder, less heat isrobbed from the solder through heat of vaporization of the solvent afterthe solder begins to melt, the effect being an amelioration of theproblem of unmelted solder powder being left behind. Because a specificamount of the second solvent remains even after the solder has melted,the fluidity of the rosin or other resin component is maintained, theactivator is not carried away as the solvent is vaporized, and theactivator is able to get into all parts of the solder, allowing it toact more effectively. As a result, it is possible to form solder bumpswith a good spherical shape and no variance in size. Also, as discussedabove, the solder powder that has already filled the openings 23 is notscraped away by repeated squeegeeing, so the amount of solder paste P′filling the openings 23 remains constant, and this also contributes toless variance in the size of the bumps.

[0153] As shown in FIGS. 5c and 5 d, the removal of the mask 2 iscarried out by dissolving away the first cover layer 21 with a treatmentliquid such as water or an aqueous solution, and then peeling the secondcover layer 22 from the substrate 1. In other words, if the second coverlayer 22 is formed from a material that does not dissolve in water, justthe first cover layer 21 will be selectively removed as shown in FIG.5c. This eliminates the adhesion between the second cover layer 22 andthe substrate 1, and allows the second cover layer 22 ⁷ to be removed asshown in FIG. 5d.

[0154] Naturally, if the first cover layer 21 ⁶ is dissolved by anaqueous solution that is also capable of dissolving the second coverlayer 22 ⁷, this water-soluble second cover layer 227 will also bedissolved by the aqueous solution, and the first and second cover layers21 and 22 can be dissolved away at the same time.

[0155] After the mask 2 is removed from the substrate 1, the substrateis separated from the substrate support 4 as shown in FIG. 5d. Since thesubstrate 1 is merely resting in the recess 40 of the substrate support4, it can be easily taken out of the substrate support 4.

[0156] With this embodiment, the mask 2 was provided by forming thecover layers 21 and 22 from resin or the like, but a metal mask in whichopenings corresponding to the electrode pads have already been made mybe used as the mask, and this mask placed over the substrate. Again whena mask such as this is used, the solder paste P used in this embodimenthas a small proportion of solder powder particles with relatively largediameter as compared to the thickness of the mask, so when the metalmask is removed after the openings have been filled with the solderpaste, there is little danger that the solder paste clinging to theinner walls of the openings will taken away along with the metal mask.Accordingly, variance in the size of the bumps is less likely to occurin a bump formation method that makes use of a metal mask.

[0157] Next, the bump formation method pertaining to the secondembodiment of the present invention will be described through referenceto FIGS. 6 and 7. FIGS. 6a to 6 f illustrate the bump formation methodpertaining to the second embodiment of the present invention, whileFIGS. 7a and 7 b are enlarged detail views of FIGS. 6a and 6 b,respectively. In this embodiment, solder bumps are formed all at once ina plurality of regions that will subsequently become individualelectronic elements, in the form of a wafer (substrate) on whichelectrode pads and wiring have been formed.

[0158] The object of the bump formation method in this embodiment is todivide the electrode pads 10 a and 10 b into a plurality of groups asshown in FIG. 6a, and form bumps Ba and Bb of different heights in eachgroup as shown in FIG. 6f. Accordingly, the size of the openings 23 aand 23 b formed in the mask 2 is different for each group, but the basicsteps are the same as in the bump formation method pertaining to thefirst embodiment of the present invention. This is described in specificterms below.

[0159] As shown in FIG. 6a, a plurality of first electrode pads 10 a anda plurality of second electrode pads 10 b are formed on the substrate 1.As shown in FIG. 7a, these electrode pads 10 a and 10 b are formed byforming an insulating film 11 on the substrate 1 so as to expose theareas that will become terminals for wiring (not shown) formed in apattern, and then performing metal plating or the like so as to coverthe exposed areas and their peripheral components. Accordingly, theelectrode pads 10 a and 10 b are shaped such that their center portionsare lower than their peripheral components. It is preferable for thefirst electrode pads 10 a to be formed in a smaller surface area thanthe second electrode pads 10 b so that their will be a good differencein the heights of the bumps Ba and Bb.

[0160] As shown in FIG. 6b, a cover layer 2A is formed on the substrate1 from a photosensitive and insulating resin material so as to cover theelectrode pads 10 a and 10 b, for instance. The cover layer 2A isformed, for example, by forming the above-mentioned resin material as afilm, and subjecting this to hot press bonding over the substrate 1. Thecover layer 2A formed in this manner conforms closely to the insulatingfilm 11, as shown in FIG. 7b. In contrast, because the electrode pads 10a and 10 b are recessed, the cover layer 2A does not conform to thecenters of the electrode pads 10 a and 10 b, and some space is left inbetween.

[0161] Examples of the resin material that makes up the cover layer 2Ainclude polymethyl methacrylate, polyacrylate, and polymethylisopropenyl ketone.

[0162] Next, just as with the first embodiment of the present invention,exposure and developing treatments are performed on the areascorresponding to the electrode pads 10 a and 10 b in the cover layer 2A.As shown in FIG. 6c, this forms openings 23 a and 23 b corresponding tothe electrode pads 10 a and 10 b in the cover layer 2A, and provides themask 2 over the substrate 1. Here, by forming windows of a specificsurface area in the photomask used for the exposure treatment, firstopenings 23 a of relatively large open surface area and volume areformed in those areas corresponding to the first electrode pads 10 a,while second openings 23 b of relatively small open surface area andvolume are formed in those areas corresponding to the second electrodepads 10 b.

[0163] The developing treatment is carried out, for example, by usingetching to remove the areas in the cover layer 2A where the openings 23a and 23 b are to be formed. If the cover layer 2A is floating above thecenters of the electrode pads 10 a and 10 b, the cover layer 2A will besuitably removed, and as a result the cover layer 2A will not remainbehind in the centers of the electrode pads 10 a and 10 b after theopenings 23 a and 23 b have been formed.

[0164] After the first and second openings 23 a and 23 b have beenformed, they are filled with solder paste Pa and Pb as shown in FIG. 6d.This filling with solder paste Pa and Pb is accomplished by the sameprinting step as described for the first embodiment of the presentinvention, for example. Naturally, the solder paste Pa and Pb can be thesame as what was used in the first embodiment of the present invention.

[0165] Next, the solder paste Pa and Pb filling the openings 23 a and 23b, respectively, is heated and melted by a heat treatment. Thiseliminates through volatilization any components such as solvent otherthan the solder component contained in the solder paste Pa and Pb, andthe solder component is gathered together into an approximate sphericalshape through surface tension as shown in FIG. 6e. The solder solidifiesin this shape in the subsequent cooling process, forming a plurality ofsolder bumps Ba and Bb affixed to the electrode pads 10 a and 10 b.Here, the first bumps Ba which are relatively taller and larger involume are formed in the first openings 23 a which have a relativelylarge volume, while the second bumps Bb which are relatively shorter andsmaller in volume are formed in the openings second openings 23 b whichhave a relatively small volume.

[0166] After this, as shown in FIG. 6f, the mask 2 is removed from thesubstrate 1 by peeling the mask 2 from the substrate 1 or dissolving themask 2 with a suitable solvent.

[0167] The substrate (wafer) 1 on which the first bumps Ba and secondbumps Bb of different heights have been formed is divided up intoindividual electronic element formation regions using a diamond cutteror the like, which provides, for example, an electronic element 7provided in its center with two regions where the shorter second bumpsBb are gathered, and in which the taller first bumps Ba are formed inthe other regions, as shown in FIG. 8.

[0168] As shown in FIG. 9, for instance, other electronic elements(sub-chips) 81 and 82 are mounted on the electronic element (main chip)7 shown in FIG. 8. These sub-chips 81 and 82 each have a surface areacorresponding to a region where the second bumps Bb are gathered, asshown in FIG. 10a, and their surfaces are provided with electrode pads81 a and 82 a that correspond to the second bumps Bb of the main chip 7and that are electrically connected to wiring (not shown). A memory LSI,analog element, or the like is used for each of the sub-chips 81 and 82.

[0169] As shown in FIG. 10b, the mounting of the sub-chips 81 and 82 iscarried out by positioning the electrode pads 81 a and 82 a of thesub-chips 81 and 82 so that they are facing and in contact with thesecond bumps Bb of the main chip, then heating and melting the shorterbumps 18, and finally cooling.

[0170] The main chip 7 on which the sub-chips 81 and 82 are mounted isitself mounted on a rewiring substrate 9 as shown in FIG. 11, forexample. The rewiring substrate 9 is provided with a plurality ofelectrode pads 90 corresponding to the first bumps Ba of the main chip 7and is electrically connected to wiring (not shown). In this mounting ofthe main chip 7 to the rewiring substrate 9, as shown in FIG. 12a,first, the main chip 7 is turned over and the first bumps Ba of the mainchip 7 are positioned with respect to the electrode pads 90 of therewiring substrate 9.

[0171] Then, as shown in FIG. 12b, the first bumps Ba of the main chip 7are brought into opposition and contact with the various electrode pads90 of the rewiring substrate 9, the first bumps Ba are heated, melted,and cooled in this state, and the main chip 7 is fixed to the rewiringsubstrate 9, thereby forming a multi-chip package X.

[0172] Here, the sub-chips 81 and 82 are held between the main chip 7and the rewiring substrate 9. To attain this state as desired, if wetake into account the deformation of the first bumps Ba when the mainchip 7 is mounted on the rewiring substrate 9, it is preferable for thedistance from the first electrode pads 10 a of the main chip 7 to thetop of the first bumps Ba to be at least 1.2 times the distance from thefirst electrode pads 10 a to the outer surface of the sub-chips 81 and82.

[0173] Next, the bump formation method pertaining to the thirdembodiment of the present invention will be described through referenceto FIG. 13. In this embodiment, solder bumps are formed all at once in aplurality of regions that will subsequently become individual electronicelements, in the form of a wafer (substrate) on which electrode pads andwiring have been formed.

[0174] The object of the bump formation method in this embodiment is todivide the electrode pads 10 a′ and 10 b′ into a plurality of groups asshown in FIG. 13a, and form bumps Ba′ and Bb′ of different heights ineach group as shown in FIG. 13h. Accordingly, the method for forming themask 2 is different from that in the bump formation methods pertainingto the first and second embodiments of the present invention, but thebasic steps are the same as in the bump formation methods pertaining tothe first and second embodiments of the present invention. This isdescribed in specific terms below.

[0175] As shown in FIG. 13a, a plurality of first electrode pads 10 a′and a plurality of second electrode pads 10 b′ are formed on thesubstrate 1′. As shown in FIG. 13b, an insulating first cover layer 21′is formed on this substrate 1′ so as to cover the electrode pads 10 a′and 10 b′. The first cover layer 21′ is formed, for example, byperforming hot press bonding on a film formed from an insulating resinmaterial. Examples of the resin that makes up the first cover layer 21′include epoxyacrylate, epoxy, and polyimide.

[0176] Next, as shown in FIG. 13c, first openings 21 a′ are formed inthe first cover layer 21′ at positions corresponding to the firstelectrode pads 10 a′ by a known technique, such as photolithography. Thefirst openings 21 a′ are formed so that their open surface area issmaller than the surface area of the first electrode pads 10 a′.Simultaneously with the formation of the first openings 21 a′, theportions of the first cover layer 21′ corresponding to the regions wherethe second electrode pads 10 b′ were formed are removed to expose thesecond electrode pads 10 b′.

[0177] Then, as shown in FIG. 13d, a second cover layer 22′ is formed soas to cover the first cover layer 21′ and the second electrode pads 10b′. The second cover layer 22′ can be formed from the same resinmaterial as the cover layer 2A formed in the second embodiment (see FIG.6b).

[0178] Next, as shown in FIG. 13e, a plurality of second and thirdopenings 22 a′ and 22 b′ are formed in the second cover layer 22 by thesame method as in the second embodiment, such as photolithography. Thesecond openings 22 b′ are formed at positions corresponding to thesecond electrode pads 10 b′, and have relatively small open surface areaand volume. The third openings 22 a′ are formed at positionscorresponding to the first electrode pads 10 a′, are electricallyconnected to the first openings 21 a′, and have a relatively larger opensurface area and volume than the first and second openings 21 a′ and 22b′.

[0179] A mask 2′ is formed by thus forming the first cover layer 21′ andthe second cover layer 22′, and forming the first to third openings 21a′, 22 a′, and 22 b′.

[0180] Then, as shown in FIG. 13f, the first to third openings 21 a′, 22a′, and 22 b′ are filled with solder paste Pa′ and Pb′. The methoddescribed for the first embodiment of the present invention can beemployed for filling with the solder paste Pa′ and Pb′.

[0181] Next, the solder paste Pa and Pb′ filling the openings 21 a′, 22a′, and 22 b′ is melted by heat treatment. This eliminates throughvolatilization any components such as solvent other than the soldercomponent contained in the solder paste Pa′ and Pb′. As shown in FIG.13g, the solder paste Pa′ filling the first openings 21 a′ is meltedwhile inside these openings. The solder component in the solder pastePa′ that fills each of the third openings 22 a′ is gathered together ina substantially spherical shape by its surface tension. In thesubsequent cooling process, this shape is maintained as the solder isaffixed on the first electrode pads 10 a′ via the solder paste Pa′filling the first openings 21 a′, resulting in the first bumps Ba′. Thesolder powder in the solder paste Pb′ filling each of the secondopenings 22 b′ is also gathered together in a substantially sphericalshape by its surface tension, and in the subsequent cooling process,this shape is maintained as the solder is affixed on the secondelectrode pads 10 b′, resulting in the second bumps Bb′.

[0182] Finally, as shown in FIG. 13h, the bumps Ba′ and Bb′ are formedon the electrode pads 10 a′ and 10 b′ of the substrate 1′ by removingthe second cover layer 22′ from the substrate 1′. Here, the first coverlayer 21′ is left behind without being removed, and the first bumps Ba′are formed such that they are raised up to the first cover layer 21′.The spherical portions of the first bumps Ba′ are larger than the secondbumps Bb′ because the third openings 22 a′ are larger in volume than thesecond openings 22 b′. As a result, the first bumps Ba′ are larger thanthe second bumps Bb′ in terms of the distance from the electrode pads 10a′ and 10 b′ to their tops.

[0183] Just as in the second embodiment, the substrate (wafer) 1′ onwhich the first bumps Ba′ and second bumps Bb′ of different heights havebeen formed is divided up into individual electronic element formationregions using a diamond cutter or the like. In this electronic element7′, just as with the electronic element 7 in the second embodiment shownin FIG. 8, the shorter second bumps Bb′ are gathered together in thecenter and the taller first bumps Ba′ are formed in the other regions,as is clearly shown in FIG. 13h.

[0184] This electronic element (main chip) 7′ can be used in amulti-chip package X′ with increased density, just as in the secondembodiment of the present invention.

[0185] In this case, first, as shown in FIGS. 14a and 14 b, anotherelectronic element (sub-chip) 8′ is mounted on the electronic element(main chip) 7′. A memory LSI, analog element, or the like is used forthis sub-chip 8′. The mounting of the sub-chip is carried out bypositioning the electrode pads 80′ provided to the sub-chip 8′ so thatthey are facing and in contact with the second bumps Bb′ of the mainchip 7′, then heating and melting the second bumps Bb′ in this state,and finally cooing.

[0186] The main chip 7′ on which the sub-chip 8′ is mounted is itselfmounted on a rewiring substrate 9′ as shown in FIGS. 14c and 14 d, forexample. In this mounting of the main chip 7′ to the rewiring substrate9′, as shown in FIG. 14c, first, the main chip 7′ is turned over and thefirst bumps Ba′ of the main chip 7′ are positioned with respect to theelectrode pads 90′ of the rewiring substrate 9′. Then, as shown in FIG.14d, the first bumps Ba′ of the main chip 7′ are brought into oppositionand contact with the various electrode pads 90′ of the rewiringsubstrate 9′, the first bumps Ba′ are heated, melted, and cooled in thisstate, and the main chip 7 is fixed to the rewiring substrate 9′,thereby forming a multi-chip package X′.

[0187] Here, the sub-chip 8′ is held between the main chip 7′ and therewiring substrate 9′. To attain this state as desired, if we take intoaccount the deformation of the first bumps Ba′ when the main chip 7′ ismounted on the rewiring substrate 9′, it is preferable for the distancefrom the first electrode pads 10 a′ of the main chip 7′ to the top ofthe first bumps Ba′ to be at least 1.2 times the distance from the firstelectrode pads 10 a′ to the outer surface of the sub-chip 8′ Next, asemiconductor chip (electronic component) 7″ of a fourth embodiment ofthe present invention, and a multi-chip package X″ that makes use ofthis chip, will be described through reference to FIGS. 15 to 17.

[0188] As shown in FIG. 15, first electrode pads 10 a″ which have arelatively small surface area, second electrode pads 10 b″ which have arelatively large surface area, and a wiring component (not shown) whichis electrically connected to these are formed on the surface of the chipsubstrate of a semiconductor chip 7″. Relatively large first bumps Ba″and relatively small second bumps Bb″ are junction-formed on the firstand second electrode pads 10 a″ and 10 b″, respectively.

[0189] The semiconductor chip 7″ can be obtained through the same stepsas in the bump formation method pertaining to the second embodiment,with the only structural difference between a difference in the surfacearea of the first electrode pads 10 a″ and the second electrode pads 10b″.

[0190] With the bump formation method pertaining to the secondembodiment, a larger amount of solder paste was placed on the firstelectrode pads 10 a″ than on the second electrode pads 10 b″ (see FIG.6d). Accordingly, this bump formation method is employed to formrelatively tall first bumps Ba″ on the first electrode pads 10 a″ andrelatively short second bumps Bb″ on the second electrode pads 10 b″.

[0191] Because the surface area of the first electrode pads 10 a″ isrelatively small, the junction surface area of the first bumps Ba″ andthe first electrode pads 10 a″ is relatively small, so the shape of thefirst bumps Ba″ is close to spherical. On the other hand, since thesurface area of the second electrode pads 10 b″ is relatively large, thejunction surface area of the second bumps Bb″ and the second electrodepads 10 b″ is relatively large, so the second bumps Bb″ have a shape inwhich a relatively large part of the sphere is missing. This means thatwhen a given amount of solder paste is placed on an electrode pad, ataller bump can be formed when the surface area of the electrode pad issmaller. Therefore, a significant difference in the heights of the firstand second bumps Ba″ and Bb″ can also be ensured by providing adifference in the surface areas of the electrode pads 10 a″ and 10 b″,as with the semiconductor chip 7″ of this embodiment.

[0192]FIG. 16 is a cross section in which sub-chips 81″ and 82″ areplaced on the semiconductor chip 7″ shown in FIG. 15. The sub-chips 81″and 82″ are each a memory IC or analog element, for example, and have astructure in which electrode pads 81 a″ and 82 a″ corresponding to thesecond bumps Bb″ of the semiconductor chip 7″, and a wiring component(not shown) electrically connected thereto, are formed. The electrodepads 81 a″ and 82 a″ of the sub-chips 81″ and 82″ are electricallyconnected to the second electrode pads 10 b″ of the semiconductor chip7″ through the second bumps Bb″. If we take into account the deformationof the first bumps Ba″ when the semiconductor chip 7″ is placed onanother mounting object, it is preferable for the distance from thefirst electrode pads 10 a″ to the top of the first bumps Ba″ to be atleast 1.2 times the distance from the second electrode pads 10 b″ to theouter surface of the sub-chips 81″ and 82″.

[0193]FIG. 17 is a cross section of a multi-chip package (electroniccomponent) X″ in which the above-mentioned semiconductor chip 7″carrying the sub-chips 81″ and 82″ is mounted on a rewiring substrate9″. The rewiring substrate 9″ has a structure in which electrode pads90″ corresponding to the first bumps Ba″ of the semiconductor chip 7″,and a wiring component (not shown) electrically connected thereto, areformed. The electrode pads 90″ of the rewiring substrate 9″ areelectrically connected to the first electrode pads 10 a″ of thesemiconductor chip 7 through the first bumps Ba″. The sub-chips 81″ and82″ carried on the semiconductor chip 7″ via the second bumps Bb″ fitbetween the semiconductor chip 7″ and the rewiring substrate 9″, whichraises the density of the multi-chip package X″.

EXAMPLES

[0194] The present invention will now be described through examples.

[0195] In Examples 1 to 7 and Comparative Examples 1 to 4 we discuss therelationship between the particle diameter of the solder powder in thesolder paste, and the variance in the height of the bumps that areformed.

Examples 1˜7

[0196] A flux vehicle was prepared by mixing 45 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol and 20 g of diethylene glycolmonobutyl ether (as solvents), 10 g of sebacic acid (an organic acid; asan activator), and 5 g of hardened castor oil (as a thixotropic agent).A solder paste was produced by kneading this flux vehicle in a weightratio of 1:9 with an Sn-3.5% Ag solder powder having the particle sizedistribution shown in Table 2. This solder paste was used to form solderbumps as below.

[0197] An acrylate film with a thickness of 50 μm was formed on a waferhaving 30 semiconductor element formation regions provided with 10,000electrode pads (70×70 μm) at a pitch of 150 μm, and openings with adiameter of 125 μm were formed at positions corresponding to theelectrode pads by exposure and developing to create a mask. Theseopenings were filled with the above-mentioned solder paste, and heatingwas performed at 260° C., which melted the solder paste and integratedthe solder to form bumps. The average bump height and the variance hereare given in Table 2. Variance in Table 2 is indicated by the standarddeviation. TABLE 2 Particle Powder Powder Powder Powder Powder PowderPowder size (μm) 1 2 3 4 5 6 7 >75 0 0 0 0 0 0 0 50 to 75 0 10 8 5 8 8 820 to 50 30 30 30 30 50 70 90 <20 70 60 62 65 42 22 2 Aver. 16.0 21.020.0 19.0 23.0 26.0 28.0 particle size (μm) Viscosity 180 195 200 220210 190 175 (Pa · s) Aver. bump 71.8 72.1 72.3 72.6 70.6 70.5 70.1.height Variance 1.5 1.5 1.7 1.8 1.5 1.8 1.9

Comparative Examples 1˜4

[0198] A flux vehicle was prepared by mixing 45 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol and 20 g of diethylene glycolmonobutyl ether (as solvents), 10 g of sebacic acid (an organic acid; asan activator), and 5 g of hardened castor oil (as a thixotropic agent).A solder paste was produced by kneading this flux vehicle in a weightratio of 1:9 with an Sn-3.5% Ag solder powder having the particle sizedistribution shown in Table 3. This solder paste was used to form solderbumps as below.

[0199] An acrylate film with a thickness of 50 μm was formed on a waferhaving 30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 150 μm, and openings witha diameter of 125 μm were formed at positions corresponding to theelectrode pads by exposure and developing to create a mask. Theseopenings were filled with the above-mentioned solder paste, and heatingwas performed at 260° C., which melted the solder paste and integratedthe solder to form bumps. The average bump height and the variance hereare given in Table 3. Variance in Table 3 is indicated by the standarddeviation. TABLE 3 Particle Powder Powder Powder Powder size (μm) 8 9 1011 >75 1 0 0 5 50 to 75 4 0 0 10 20 to 50 25 20 0 30 <20 70 80 100 55Aver. 17.0 14.5 12.5 21.5 particle size (μm) Viscosity 230 250 260 190(Pa · s) Aver. bump 71.5 71.8 72.0 69.8 height Variance 3.3 3.4 3.6 4.2

[0200] As can be seen from Tables 2 and 3, it is preferable for thesolder powder to be such that the proportion of particles whose diameter(50 to 75 μm) is at least the thickness of the mask but no more than 1.5times the thickness of the mask is no more than 10 wt %, the proportionof particles whose diameter (>50 μm) is at least 40% of the opendiameter of the openings is no more than 10 wt %, and the proportion ofparticles whose diameter (20 to 50 μm) is 40 to 100% of the maskthickness is at least 30 wt %.

[0201] In Examples 8 to 11 and Comparative Examples 5 to 8, we willexamine the variance in the height of the bumps that are formed as afunction of the type of solvent used for the solder paste.

Example 8

[0202] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol (as a first solvent), 20 g ofdiethylene glycol monobutyl ether (as a second solvent), and 10 g ofsebacic acid (an organic acid; as an activator). A solder paste wasproduced by kneading this flux vehicle in a weight ratio of 1:9 with anSn-3.5% Ag solder powder with an average particle size of 16 um (powder1 in Table 2). This solder paste was used to form solder bumps as below.

[0203] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.,which melted the solder paste and integrated the solder to form bumps.

[0204] The bumps thus formed were approximately 80 μm tall, and thedifference between the maximum and minimum height (hereinafter referredto as “variance”) was 1.2 μm, making these bumps very precise. Theresidue of halogen elements and alkali metal elements in the bumps afterthis bump formation was 10 ppm or less, and no effect on thesemiconductor elements was seen.

Example 9

[0205] A flux vehicle was prepared by mixing 55 g of Polypale (asrosin), 15 g of 2-methyl-2,4-pentanediol (as a first solvent), 20 g ofdiethylene glycol monobutyl ether (as a second solvent), and 10 g ofsuccinic acid (an organic acid; as an activator). A solder paste wasproduced by kneading this flux vehicle in a weight ratio of 1:9 with anSn-3.5% Ag solder powder with an average particle size of 16 μm (powder1 in Table 2). This solder paste was used to form solder bumps as below.

[0206] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.,which melted the solder paste and integrated the solder to form bumps.

[0207] The bumps thus formed were approximately 78 μm tall, and thevariance was 1.3 μm, making these bumps very precise. The residue ofhalogen elements and alkali metal elements in the bumps after this bumpformation was −10 ppm or less, and no effect on the semiconductorelements was seen.

Example 10

[0208] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol (as a first solvent), 20 g ofdiethylene glycol monobutyl ether (as a second solvent), and 5 g ofsebacic acid and 5 g of succinic acid (organic acids; as activators). Asolder paste was produced by kneading this flux vehicle in a weightratio of 1:9 with an Sn-3.5% Ag solder powder with an average particlesize of 16 μm (powder 1 in Table 2). This solder paste was used to formsolder bumps as below.

[0209] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.,which melted the solder paste and integrated the solder to form bumps.

[0210] The bumps thus formed were approximately 81 μm tall, and thevariance was 1.8 μm, making these bumps very precise. The residue ofhalogen elements and alkali metal elements in the bumps after this bumpformation was 10 ppm or less, and no effect on the semiconductorelements was seen.

Example 11

[0211] A flux vehicle was prepared by mixing 45 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol (as a first solvent), 20 g ofdiethylene glycol monobutyl ether (as a second solvent), and 10 g ofsebacic acid (an organic acid) and 5 g of triethanolamine (an organicamine) (as activators) A solder paste was produced by kneading this fluxvehicle in a weight ratio of 1:9 with an Sn-3.5% Ag solder powder withan average particle size of 16 μm (powder 1 in Table 2). This solderpaste was used to form solder bumps as below.

[0212] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.,which melted the solder paste and integrated the solder to form bumps.

[0213] The bumps thus formed were approximately 80 μm tall, and thevariance was 1.4 μm, making these bumps very precise. The residue ofhalogen elements and alkali metal elements in the bumps after this bumpformation was 10 ppm or less, and no effect on the semiconductorelements was seen.

Comparative Example 5

[0214] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 40 g of just 2-methyl-2,4-pentanediol (as a solvent which has aboiling point lower than the melting point of the solder powder), and 10g of sebacic acid (an organic acid; as an activator). A solder paste wasproduced by kneading this flux vehicle in a weight ratio of 1:9 with anSn-3.5% Ag solder powder with an average particle size of 16 μm (powder1 in Table 2). This solder paste was used to form solder bumps as below.

[0215] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.

[0216] The result was a solder that produced solder balls in somelocations, etc., and could not be integrated when molten. The bumps thusformed averaged about 59 μm in height, and the variance was 9 μm, whichis poor precision.

Comparative Example 6

[0217] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 20 g of 2-methyl-2,4-pentanediol and 20 g of ethylene glycoldibutyl ether (as solvents which have a boiling point lower than themelting point of the solder powder), and 10 g of sebacic acid (anorganic acid). A solder paste was produced by kneading this flux vehiclein a weight ratio of 1:9 with an Sn-3.5% Ag solder powder with anaverage particle size of 16 μm. This solder paste was used to formsolder bumps as below.

[0218] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm thick) having openings with a diameter of 160 μm atpositions corresponding to the electrode pads of these semiconductorelements was put in place after being aligned with these semiconductorelements, and the above-mentioned solder paste was printed onto theelectrode pads of the wafer. Next, heating was performed at 260° C.

[0219] The result was a solder that produced solder balls in somelocations, etc., and could not be integrated when molten. The bumps thusformed averaged about 50 μm in height, and the variance was 12 μm, whichis poor precision.

Comparative Example 7

[0220] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 40 g of diethylene glycol monobutyl ether (as a solvent whichhas a boiling point higher than the melting point of the solder powder),and 10 g of sebacic acid (an organic acid; as an activator). A solderpaste was produced by kneading this flux vehicle in a weight ratio of1:9 with an Sn-3.5% Ag solder powder with an average particle size of 16μm (powder 1 in Table 2). This solder paste was used to form solderbumps as below.

[0221] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm) having openings with a diameter of 160 μm at positionscorresponding to the electrode pads of these semiconductor elements wasput in place after being aligned with these semiconductor elements, andthe above-mentioned solder paste was printed onto the electrode pads ofthe wafer. Next, heating was performed at 260° C.

[0222] The result was that solder powder which was unmelted andtherefore not integrated remained on the bump surfaces, and the desiredbumps could not be formed.

Comparative Example 8

[0223] A flux vehicle was prepared by mixing 50 g of Polypale (asrosin), 20 g of 1,5-pentanediol and 20 g of diethylene glycol monobutylether (as solvents which have a boiling point higher than the meltingpoint of the solder powder), and 10 g of sebacic acid (an organic acid;as an activator). A solder paste was produced by kneading this fluxvehicle in a weight ratio of 1:9 with an Sn-3.5% Ag solder powder withan average particle size of 16 μm (powder 1 in Table 2). This solderpaste was used to form solder bumps as below.

[0224] Solder bumps were formed by metal mask printing on a wafer having30 semiconductor element formation regions provided with 10,000electrode pads (70 μm diameter) at a pitch of 200 μm. First, a metalmask (40 μm) having openings with a diameter of 160 μm at positionscorresponding to the electrode pads of these semiconductor elements wasput in place after being aligned with these semiconductor elements, andthe above-mentioned solder paste was printed onto the electrode pads ofthe wafer. Next, heating was performed at 260° C.

[0225] The result was that solder powder which was unmelted andtherefore not integrated remained on the bump surfaces, and the desiredbumps could not be formed.

[0226] In Examples 12 to 20, we will discuss the bump formation methodpertaining to the first embodiment described previously.

Example 12

[0227] In this example, bumps were formed by the following method.First, a silicon wafer with a thickness of 0.6 mm and a diameter of 6inches (approximately 15.3 cm), and having 30 semiconductor elementformation regions provided with 10,000 electrode pads (70 μm diameter)at a pitch of 150 μm, was coated by spin coating with an aqueoussolution containing 5.0 wt % polyvinyl alcohol, after which this washeated for 30 minutes at about 110° C. to form a first cover layer witha thickness of about 0.1 μm. A photosensitive acrylate resin film with athickness of 50 μm (trade name: NIT-250, made by Nichigo Morton) wasapplied by hot press bonding at 105° C. and 3.5 kg/mm² to form a secondcover layer.

[0228] This second cover layer was exposed through a mask in which theopaque portion was formed at positions corresponding to the electrodepads, and the region irradiated with light was polymerized. The exposedsecond cover layer was developed with an aqueous solution (etchant)containing 2.3 vol % tetramethylammonium hydroxide, which removed theportions not irradiated with light and created openings with a diameterof 125 μm. Here, because the previous etchant was an aqueous solutionand the first cover layer was formed from a water-soluble resin, theportions corresponding to the electrode pads in the first cover layerwere also removed at the same time, and openings were also formed inthis first cover layer, exposing the electrode pads.

[0229] After this, the openings in the first and second cover layerswere filled by printing with an Sn-3.5% Ag solder powder with an averageparticle size of 16 μm (powder 1 in Table 2), and the solder wassolidified after reflow at 265° C. Next, the first and second coverlayers were each dissolved away with a 5.0 vol % monoethanolamineaqueous solution. As a result, bumps 75±1.5 μm in height were formed.

Example 13

[0230] In this example, bumps were formed in the same manner as inExample 12, except that an aqueous solution containing 5.0 wt %polyacrylic acid was applied by spin coating, after which this coatingwas heated for 30 minutes at about 110° C. to form a first cover layerwith a thickness of about 0.1 μm. As a result, bumps 75±1.5 μm in heightwere formed.

Example 14

[0231] In this example, first and second cover layers were formed andopenings were formed in these cover layers in the same manner as inExample 12. After this, the insides of the openings were coated withflux, then filled with an Sn-3.5% Ag solder powder with an averageparticle size of 15 μm (powder 1 in Table 2), and this solder powder wassolidified after reflow at 265° C. Next, the first and second coverlayers were each dissolved away with a 5.0 vol % monoethanolamineaqueous solution. As a result, bumps 80±2.0 μm in height were formed.

Example 15

[0232] In this example, first and second cover layers were formed andopenings were formed in these cover layers in the same manner as inExample 12. The thickness of the second cover layer, however, waschanged to 25 μm. After this, the insides of the through holes werecoated with flux, and the substrate was dipped in a 280° C. moltensolder bath (Sn-3.5% Ag) The first and second cover layers were eachdissolved away with a 5.0 vol % monoethanolamine aqueous solution. As aresult, bumps 75±2.5 μm in height were formed.

Example 16

[0233] In this example, first and second cover layers were formed andopenings were formed in these cover layers in the same manner as inExample 15. After this, the insides of the openings were filled withsolder (Sn-3.5% Ag) by plating and then coated with flux, and this wassolidified after reflow at 265° C. The first and second cover layerswere each dissolved away with a 5.0 vol % monoethanolamine aqueoussolution. As a result, bumps 75±1.0 μm in height were formed.

Example 17

[0234] In this example, bumps were formed by the following method.First, a silicon wafer with a thickness of 0.6 mm and a diameter of 6inches (approximately 15.3 cm), and on which a total of 300,000 circularelectrodes with a diameter of 70 μm had been formed at a pitch of 150μm, was placed as a substrate on a Teflon substrate support (200×200×2mm).

[0235] Next, a photosensitive acrylate resin film (trade name: NIT-250,made by Nichigo Morton) was applied by hot press bonding at 100° C. and3.5 kg/mm² to form a cover layer. This cover layer was exposed through amask in which the opaque portion was formed at positions correspondingto the electrode pads, and the region irradiated with light waspolymerized. The exposed cover layer was developed with an aqueoussolution (etchant) containing 2.3 vol % tetramethylammonium hydroxide,which removed the portions not irradiated with light, formed openingswith a diameter of 125 μm, and created a mask.

[0236] After this, a plate with a thickness of 0.6 mm and having anopening of substantially the same shape as the silicon wafer wasdisposed so as to surround the substrate. The openings in the coverlayer were then filled by printing with a solder paste (in which thesolder powder was an Sn-3.5% Ag solder powder with an average particlesize of 16 μm (powder 1 in Table 2)), after which the plate was takenaway and the solder paste was solidified after reflow at 265° C. Thefirst and second cover layers were dissolved away with a 5.0 vol %monoethanolamine aqueous solution. As a result, bumps 75±1.5 μm inheight were formed.

Example 18

[0237] In this example, a silicon wafer was fixed to a substrate supporthaving an opening corresponding to the plan view surface area of thesilicon wafer, and in which a recess 0.45 mm deep was formed, such thatthe lower part of this wafer fit into this recess. In this state, acover layer was formed in the same manner as in Example 17, and openingswere formed in this cover layer in a pattern corresponding to theelectrode pads of the substrate, creating a mask.

[0238] Next, a plate with a thickness of 0.15 mm and having an openingof substantially the same shape as the silicon wafer was disposed so asto surround the substrate. The openings were then filled with solder,the solder was solidified after reflow, and the cover layer was removed,all in the same manner as in Example 17. As a result, bumps 75±1.5 μm inheight were formed.

Example 19

[0239] In this example, other than using a substrate support made fromstainless steel, bumps were formed in the same manner as in Example 17.As a result, bumps 75±1.5 μm in height were formed.

Example 20

[0240] In this example, other than using a substrate support made fromstainless steel, bumps were formed in the same manner as in Example 18.As a result, bumps 75+1.5 μm in height were formed.

[0241] In Examples 21 to 23, we will discuss the bump formation methodpertaining to the second embodiment of the present invention.

Example 21

[0242] 800 first electrode pads (used for electrical connection to arewiring substrate; electrode diameter: 80 μm; pitch: 220 μm) and 100×2second electrode pads (used for electrical connection to sub-chips;electrode diameter: 110 μm; pitch: 220 μm) were formed on the surface ofa chip substrate, and the resulting semiconductor chip was used as amain chip.

[0243] A photosensitive polymethyl methacrylate insulating film(thickness: 100 μm) was laid over this main chip so as to cover thefirst and second electrode pads. Exposure and developing were carriedout to form first openings at places corresponding to the firstelectrode pads in this film, and form second openings at placescorresponding to the second electrode pads, creating a mask. The opendiameter of the first openings was 200 μm, while that of the secondopenings was 50 μm. The first and second openings were filled withsolder paste containing an Sn-3.5% Ag solder powder with an averageparticle size of 16 μm (powder 1 in Table 2) in an amount of 55 vol %,after which this was heated at 260° C. to melt and integrate the solderpowder in the solder paste.

[0244] Next, the insulating film was chemically removed using a 10 wt %monoethanolamine aqueous solution. After this, the bumps were coatedwith a flux containing 50 wt % Polypale (as rosin) and 50 wt % hexyleneglycol (as a solvent), and this was heated again at 260° C. to adjustthe shape of the bumps.

[0245] As a result, first bumps 161 μm tall were formed on the firstelectrode pads used for electrical connection to the rewiring substrate,while second bumps 30 μm tall were formed on the second electrode padsused for electrical connection to the sub-chips.

[0246] Next, two semiconductor chips (thickness: 100 μm) serving assub-chips comprising 100 electrode pads on a chip substrate were placedon the second bump group of the main chip, that is, the short bump groupin which the height was 30 μm (100 of these bumps), while being heatedat 260° C. This main chip was then turned over and placed on a rewiringsubstrate via the first bumps, that is, the tall bumps with a height of161 μm, again while being heated at 260° C.

[0247] As a result, a good connection was formed between the main chipand the rewiring substrate, while the two sub-chips were held betweenthe main chip and the rewiring substrate.

Example 22

[0248] 800 first electrode pads (used for electrical connection to arewiring substrate; electrode diameter: 100 μm; pitch: 300 μm) and 100×2second electrode pads (used for electrical connection to sub-chips;electrode diameter: 80 μm; pitch: 153 μm) were formed on the surface ofa chip substrate, and the resulting semiconductor chip was used as amain chip. A photosensitive polymethyl methacrylate insulating film(thickness: 50 μm) was laid over this main chip so as to cover the firstand second electrode pads. Exposure and developing were carried out toform first and second openings at places corresponding to the variouselectrode pads in this film. The open diameter of the first openingsover the first electrode pads was 280 μm, while that of the secondopenings over the second electrode pads was 50 μm.

[0249] The first and second openings were filled with solder pastecontaining an Sn-3.5% Ag solder powder with an average particle size of16 μm (powder 1 in Table 2) in an amount of 55 vol %, after which thiswas heated at 260° C. to melt and integrate the solder powder in thesolder paste. Next, the insulating film was chemically removed using a10 wt % monoethanolamine aqueous solution. After this, the bumps werecoated with a flux containing 50 wt % Polypale (as rosin) and 50 wt %hexylene glycol (as a solvent) and this was heated again at 260° C. toadjust the shape of the bumps.

[0250] As a result, bumps 154 μm tall were formed on the first electrodepads used for electrical connection to the rewiring substrate, whilebumps 28 μm tall were formed on the second electrode pads used forelectrical connection to the sub-chips.

[0251] Next, two semiconductor chips (thickness: 100 μm) serving assub-chips comprising 100 electrode pads on a chip substrate were placedon the second bump group of the main chip, that is, the short bump groupin which the height was 28 μm (100 of these bumps), while being heatedat 260° C. This main chip was then turned over and placed on a rewiringsubstrate via the first bumps, that is, the tall bumps with a height of154 μm, again while being heated at 260° C.

[0252] As a result, a good connection was formed between the main chipand the rewiring substrate, while the two sub-chips were held betweenthe main chip and the rewiring substrate.

Example 23

[0253] 800 first electrode pads (used for electrical connection to arewiring substrate; electrode diameter: 100 μm; pitch: 300 μm) and 100×2second electrode pads (used for electrical connection to sub-chips;electrode diameter: 80 μm; pitch: 153 μm) were formed on the surface ofa chip substrate, and the resulting semiconductor chip was used as amain chip.

[0254] A photosensitive polymethyl methacrylate insulating film(thickness: 50 μm) was laid over this main chip so as to cover theelectrode pads. Exposure and developing were carried out to first andsecond openings at places corresponding to the first and secondelectrode pads in this film. The open diameter of the first openingsover the first electrode pads was 280 μm, the open diameter of thenumber 2-1 openings over the first group of second electrode pads (100pads) was 50 μm, and the open diameter of the number 2-2 openings overthe second group of second electrode pads (100 pads) was 40 μm.

[0255] The various openings were filled with solder paste containing anSn-3.5% Ag solder powder with an average particle size of 16 μm (powder1 in Table 2) in an amount of 55 vol %, after which this was heated at260° C. to melt and integrate the solder powder in the solder paste.Next, the insulating film was chemically removed using a 10 wt %monoethanolamine aqueous solution. After this, the bumps were coatedwith a flux containing 50 wt % Polypale (as rosin) and 50 wt % hexyleneglycol (as a solvent), and this was heated again at 260° C. to adjustthe shape of the bumps.

[0256] As a result, the first bumps 154 μm tall were formed on the firstelectrode pads used for electrical connection to the rewiring substrate,while bumps 28 μm and 18 μm tall (numbers 2-1 and 2-2) were respectivelyformed on the first and second groups of second electrode pads used forelectrical connection to the sub-chips.

[0257] Next, a first semiconductor chip (thickness: 100 μm) and a secondsemiconductor chip (thickness: 105 μm) serving as sub-chips comprising100 electrode pads on a chip substrate were respectively placed on thenumber 2-1 bumps of the main chip, that is, the bumps 28 μm tall (100 ofthese bumps), and on the number 2-2 bumps, that is the bumps 18 μm tall(100 of these bumps), while being heated at 260° C. This main chip wasthen turned over and placed on a rewiring substrate via the first bumps,that is, the tall bumps with a height of 154 μm, again while beingheated at 260° C.

[0258] As a result, a good connection was formed between the main chipand the rewiring substrate, while the two sub-chips were held betweenthe main chip and the(rewiring substrate.

What is claimed is:
 1. A bump formation method in which bumps are formedon a substrate provided with a plurality of electrode pads, comprisingthe steps of: providing a mask having a plurality of openingscorresponding to the plurality of electrode pads; filling each of theopenings with a solder paste; and heat treating the solder paste,wherein the solder paste contains solder powder and a flux vehicle, andthe solder powder contains no more than 10 wt % particles whose diameteris greater than the thickness of the mask and no more than 1.5 timesthis thickness.
 2. A bump formation method as claimed in claim 1,wherein the solder powder contains no more than 10 wt % particles whosediameter is no less than 40% of the diameter of the openings.
 3. A bumpformation method as claimed in claim 1, wherein the solder powdercontains no more than 30 wt % particles whose diameter is 40 to 100% thethickness of the mask.
 4. A bump formation method as claimed in claim 1,wherein the average particle diameter of the solder powder is 5 to 20μm.
 5. A bump formation method as claimed in claim 1, wherein the fluxvehicle contains a first solvent having a boiling point lower than themelting point of the solder powder, and a second solvent having aboiling point higher than the melting point of the solder powder.
 6. Abump formation method as claimed in claim 5, wherein the boiling pointof the first solvent is 5 to 50° C. lower than the melting point of thesolder powder, and the boiling point of the second solvent is 5 to 50°C. higher than the melting point of the solder powder.
 7. A bumpformation method as claimed in claim 5, wherein the first solvent iscontained in the solder paste in an amount of 2 to 6 wt %, and thesecond solvent is contained in an amount of 2 to 6 wt %.
 8. A bumpformation method as claimed in claim 1, wherein the total content ofhalogen elements and alkali metal elements in the flux vehicle is nomore than 100 ppm.
 9. A bump formation method as claimed in claim 1,wherein the flux vehicle further contains an activator, and thisactivator contains at least one type of organic acid or organic amineselected from the group consisting of sebacic acid, succinic acid,adipic acid, glutaric acid, triethanolamine, and monoethanolamine.
 10. Abump formation method as claimed in claim 9, wherein the activator iscontained in the solder paste in an amount of 0.01 to 2 wt %.
 11. A bumpformation method as claimed in claim 1, wherein the viscosity of thesolder paste is 100 to 400 Pa·s.
 12. A bump formation method as claimedin claim 1, wherein the mask is provided over the substrate through thesteps of forming a first cover layer over the substrate, forming asecond cover layer over this first cover layer, and forming theplurality of openings in the first cover layer and the second coverlayer by exposing these to light in a pattern corresponding to theplurality of electrode pads and developing with an etchant, and thefirst cover layer is formed from a material that will be dissolved bythe etchant used to develop the second cover layer, with the etching ofthe first cover layer being carried out simultaneously with thedeveloping of the second cover layer.
 13. A bump formation method asclaimed in claim 12, wherein the first cover layer is formed from amaterial containing a macromolecule that is water-soluble or readilydissolves in an alkaline aqueous solution.
 14. A bump formation methodas claimed in claim 1, wherein the plurality of electrode pads aredivided into a plurality of groups, and the mask is formed through thesteps of forming a cover layer so as to cover the plurality of electrodepads, and forming the plurality of openings in this cover layer in apattern corresponding to the plurality of electrode pads, with thevolume of these openings being different for each group.
 15. A bumpformation method as claimed in claim 14, wherein the cover layer isformed by laying a resin film over the substrate.
 16. A bump formationmethod as claimed in claim 14, wherein the cover layer contains at leastone type of component selected from the group consisting of polymethylmethacrylate, polyacrylate, and polymethyl isopropenyl ketone.
 17. Abump formation method as claimed in claim 14, wherein the plurality ofelectrode pads are divided into a group comprising a plurality of firstelectrode pads and a group comprising a plurality of second electrodepads, each of the first electrode pads being formed in a surface areasmaller than each of the second electrode pads, and the plurality ofopenings include a plurality of first openings formed in a patterncorresponding to the plurality of first electrode pads, and a pluralityof second openings each smaller in volume than each of the firstopenings and formed in a pattern corresponding to the plurality ofsecond electrode pads.
 18. A bump formation method as claimed in claim1, wherein the plurality of electrode pads include a plurality of firstelectrode pads and a plurality of second electrode pads, and theplurality of openings include a plurality of first openings, a pluralityof second openings, and a plurality of third openings, and the mask isformed through the steps of forming a first cover layer by covering theplurality of first electrode pads and exposing the plurality of secondelectrode pads, forming the plurality of first openings in this firstcover layer in a pattern corresponding to the plurality of firstelectrode pads, forming a second cover layer so as to cover the firstcover layer and the plurality of second electrode pads, forming theplurality of second openings in the second cover layer in a patterncorresponding to the plurality of second electrode pads, and forming theplurality of third openings in a pattern corresponding to the pluralityof first openings.
 19. A bump formation method as claimed in claim 18,wherein each of the first openings is formed with a larger open surfacearea than each of the second openings.
 20. A bump formation method asclaimed in claim 18, wherein each of the third openings is formed with alarger open surface area than each of the first openings, there isfurther included a step of selectively removing just the second coverlayer, and the first cover layer is left on the substrate.
 21. A bumpformation method as claimed in claim 18, wherein the first cover layeror the second cover layer, or both, is or are formed by laying a resinfilm over the substrate.
 22. A bump formation method as claimed in claim18, wherein the first cover layer contains at least one type of compoundselected from the group consisting of epoxyacrylate, epoxy, andpolyimide.
 23. A bump formation method as claimed in claim 18, whereinthe second cover layer contains at least one type of compound selectedfrom the group consisting of polymethyl methacrylate, polyacrylate, andpolymethyl isopropenyl ketone.
 24. A bump formation method as claimed inclaim 1 wherein the filling of the openings with solder paste is carriedout through the steps of holding the substrate on a substrate support,providing squeegeeing helper means for lessening the difference betweenthe height position of the mask and the height position of the peripheryof the substrate, readying solder paste on the mask or the squeegeeinghelper means, and moving a squeegee to push the solder paste down intothe openings.
 25. A bump formation method as claimed in claim 24,wherein the height position of the periphery of the substrate is made bythe squeegeeing helper means to be the same or substantially the same asthe height position of the cover layer.
 26. A bump formation method asclaimed in claim 24, wherein the squeegeeing helper means is a platehaving an opening corresponding to the shape of the substrate.
 27. Abump formation method as claimed in claim 24, wherein the substratesupport has a recess capable of accommodating at least part of thesubstrate.
 28. A bump formation method as claimed in claim 1, furthercomprising a step of applying flux to the bumps formed from heat treatedsolder paste, and performing a heat treatment again to adjust the shapeof the bumps.
 29. A bump formation method as claimed in claim 28,wherein the flux contains Polypale and hexylene glycol.
 30. A bumpformation method as claimed in claim 1, wherein the open surface area ofeach of the openings is no more than 25 times the surface area of thecorresponding electrode pad.
 31. A bump formation method for formingbumps on a substrate provided with a plurality of electrode pads,comprising the steps of: forming a first cover layer over the substrate;forming a second cover layer over the first cover layer; forming aplurality of openings corresponding to the plurality of electrode padsin the first cover layer and the second cover layer by exposing these tolight and developing with an etchant; filling each of the openings withmetal; and heating the metal to integrate it with the electrode pads,wherein the first cover layer is formed from a material that will bedissolved by the etchant used to develop the second cover layer, and thefirst cover layer is etched to form the plurality of openingssimultaneously with the developing of the second cover layer.
 32. A bumpformation method for forming bumps on a substrate provided with aplurality of electrode pads divided into a plurality of groups,comprising the steps of: forming a mask having a plurality of openingscorresponding to the plurality of electrode pads such that the size isdifferent for each group; filling the openings with solder paste;forming bumps from the solder paste by heat treatment; and removing thecover layer from the substrate.
 33. A bump formation method for formingbumps on a substrate provided with a plurality of first electrode padsand a plurality of second electrode pads, comprising the steps of:forming a first cover layer in a state in which the plurality of firstelectrode pads are covered and the plurality of second electrode padsare exposed; forming a plurality of first openings in the first coverlayer in a pattern corresponding to the plurality of first electrodepads; forming a second cover layer so as to cover the first cover layerand the plurality of second electrode pads; forming a plurality ofsecond openings in the second cover layer in a pattern corresponding tothe plurality of second electrode pads, and forming a plurality of thirdopenings in a pattern corresponding to the plurality of first openings;filling the first openings, second openings, and third openings withsolder paste; and forming bumps from the solder paste by heat treatment.34. A bump formation method for forming bumps on a substrate providedwith a plurality of electrode pads, comprising the steps of: holding thesubstrate on a substrate support; forming a cover layer so as to coverat least the substrate; forming a plurality of openings in the coverlayer in a pattern corresponding to the plurality of electrode pads;providing squeegeeing helper means for lessening the difference betweenthe height position of the cover layer on the substrate and the heightposition of the periphery of the substrate; readying a metal paste ormetal powder on the cover layer or the squeegeeing helper means; movinga squeegee to push the metal paste or metal powder down into theopenings; heating, melting, and solidifying the metal paste or metalpowder to integrate the same on the electrode pads; and taking away thesqueegeeing helper means.
 35. An electronic component, comprising: asubstrate; a plurality of first electrode pads and a plurality of secondelectrode pads formed on the same surface of this substrate; a pluralityof first bumps formed in a pattern corresponding to the plurality offirst electrode pads; and a plurality of second bumps formed in apattern corresponding to the plurality of second electrode pads, whereinthe surface area of each of the first electrode pads is smaller than thesurface area of each of the second electrode pads, and the top of eachof the first bumps is located higher than the top of each of the secondbumps.
 36. An electronic component as claimed in claim 35, furthercomprising a mounting object, wherein this mounting object is placed onthe substrate with the plurality of second bumps therebetween, and thetop of each of the first bumps is located at a height of at least 1.2times the height location of the top of the mounting object.
 37. Anelectronic component as claimed in claim 36, further comprising anadditional mounting object, wherein this additional mounting object isplaced on the substrate via the plurality of first bumps in a state inwhich the original mounting object is interposed between the additionalmounting object and the substrate.
 38. An electronic component asclaimed in claim 36, wherein the substrate is mounted on anothersubstrate via the plurality of first bumps in a state in which themounting object is interposed between the original substrate and theother substrate.
 39. An electronic component, comprising: a substrate; aplurality of first electrode pads and a plurality of second electrodepads formed on the same surface of this substrate; a cover layer formedin the region of the substrate where the plurality of first electrodepads are formed, and having a plurality of openings corresponding to theplurality of first electrode pads; a plurality of first bumps providedin a pattern corresponding to the plurality of first electrode pads,with spherical portions protruding from the cover layer; and a pluralityof second bumps provided in a pattern corresponding to the plurality ofsecond electrode pads, with spherical portions formed directly on thecorresponding second electrode pads, wherein the top of each of thefirst bumps is located higher than the top of each of the second bumps.40. An electronic component as claimed in claim 39, wherein the coverlayer contains at least one type of resin selected from the groupconsisting of epoxyacrylate, epoxy, and polyimide.
 41. An electroniccomponent as claimed in claim 39, further comprising a mounting object,wherein this mounting object is placed on the substrate with theplurality of second bumps therebetween, and the top of each of the firstbumps is located at a height of at least 1.2 times the height locationof the top of the mounting object.
 42. An electronic component asclaimed in claim 41, further comprising an additional mounting object,wherein this additional mounting object is placed on the substrate viathe plurality of first bumps in a state in which the original mountingobject is interposed between the additional mounting object and thesubstrate.
 43. An electronic component as claimed in claim 41, whereinthe substrate is mounted on another substrate via the plurality of firstbumps in a state in which the mounting object is interposed between theoriginal substrate and the other substrate.
 44. A solder pastecontaining a solder powder and a solvent, wherein the solvent contains afirst solvent having a boiling point lower than the melting point of thesolder powder, and a second solvent having a boiling point higher thanthe melting point of the solder powder.
 45. A solder paste as claimed inclaim 44, wherein the boiling point of the first solvent is 5 to 50° C.lower than the melting point of the solder powder, and the boiling pointof the second solvent is 5 to 50° C. higher than the melting point ofthe solder powder.
 46. A solder paste as claimed in claim 44, whereinthe first solvent is contained in the solder paste in an amount of 2 to6 wt %, and the second solvent is contained in an amount of 2 to 6 wt %.